Cubone is only found in Glittering Cave in both games.
Cubone is one of the rarest Pokémon to get in any nuzlocke. In Kanto games, it is only available in the Pokémon Tower, except in Yellow (Safari Zone); in Johto games, it can only be caught in Rock Tunnel, with the exception of Crystal (Game Corner prize) and the remakes (Safari Zone). In all other regions except Kalos, it cannot be found before the postgame. In all games it is found in, it is typically rare. This trend is ironically well reflected in Cubone's loneliness.
There are also not many reasons to leave this Pokémon in its lonely state. While fairly average as a Ground-type overall, especially with the scarcity of Thick Clubs available - unless patient enough to hunt for one in Glittering Cave by repeatedly Frisking wild Cubone - Marowak will likely not deal a lot of damage for a lot of the time. It is a more defensive Pokémon than not, but in that department, it does the job really well; its pure typing also helps it avoid the common problem of a double weakness to Grass and/or Water, making some attacks from those types actually tankable. Naturally, Marowak should not be the designated counter for the aforementioned types either, but its risk of dying is considerably lesser. While not awfully advantaged when compared to the likes of Krookodile and Hippowdon, Cubone can still hold its own against some of the key threats of the game, including Wikstrom's Aegislash.
Gym #2 - Grant (Cyllage City, Rock-type): Cubone can survive a Refrigerate-boosted Take Down from Amaura, but its Bonemerang has a very small chance of KOing in one turn, and if that does not happen, Cubone will need to switch out. Cubone's health is best saved for Tyrunt instead, if there are better Amaura counters; Bonemerang KOs Tyrunt in two turns and even its Strong Jaw-boosted Bite will only barely leave a dent.
Korrina (Geosenge Town, Fighting-type): This battle is dangerous for Cubone; while Bonemerang 2HKOs both Lucario, a single miss will cost Cubone a switch, because Power-Up Punch 3HKOs counting the Attack boosts Lucario receives. Marowak, instead, OHKOs both Lucario like a champ.
Rival (Tower of Mastery): Marowak's Bonemerang 2-3HKOs Meowstic, which can only 4HKO at best, and also 2HKOs Absol. It can OHKO Braixen as well, but Quilladin and Frogadier will both beat it with their respective STABs. The only way for Marowak to win against them is to set up Swords Dances against Absol; at +2 in Attack, it can OHKO Frogadier if its health is above half and Water Pulse can be survived. Quilladin is 2HKOed by Aerial Ace at +2; this is fine if Marowak is kept healthy, since its Needle Arm also fails to 2HKO from full.
Gym #3 - Korrina (Shalour City, Fighting-type): Mienfoo can be 2HKOed by Marowak's Dig as well as Bonemerang, and Dig is safer to use, due to having higher accuracy. Using Swords Dance and then a Ground move accomplishes the same, and is the recommended option, as it will simplify Marowak's job against Machoke and Hawlucha. Machoke is OHKOed at +2, but only by Bonemerang; Marowak is highly unlikely to miss Bonemerang enough times for Machoke's Power-Up Punch to become a problem, though, especially since even a +6 Power-Up Punch is only a 2-3HKO. Marowak also wins against Hawlucha after a Swords Dance, 2HKOing it with Aerial Ace.
Rival (Coumarine City): Marowak can profit of Meowstic to use Swords Dance twice and then OHKO with Bonemerang or Dig, as it easily survives up to three Psybeams; it will need to be healed after setting up, however. At +4, Marowak's Ground STAB destroys Absol and Braixen, as well as Frogadier; Marowak needs to be well above half health to survive a Water Pulse from the latter, though. Aerial Ace 1-2HKOs Quilladin at the same level of boosts.
Gym #4 - Ramos (Coumarine City, Grass-type): Marowak can beat Jumpluff with three consecutive Rock Tombs if its Speed is high enough; after Jumpluff's Speed is halved, Marowak has a decent chance of outspeeding, but will be knocked out by Grass Knot if this does not happen. Weepinbell and Gogoat will 2HKO it with Grass Knot; avoid them.
Aliana (Kalos Power Plant): Mightyena is fairly threatening with Swagger and Assurance combined; Assurance can chip away half of its health after it hits itself in confusion. Marowak's best option is to use Rock Tomb until it outspeeds Mightyena, then pay close attention to its Attack boosts if confused; at half health or below, Bonemerang is likely to seal the KO.
Gym #5 - Clemont (Lumiose City, Electric-type): Marowak's immunity to Electric moves is very useful, as it prevents Emolga from switching out with Volt Switch. This enables Marowak to use a Swords Dance undisturbed, and may also go for two if needed; Rock Tomb can OHKO Emolga after just one, but is not guaranteed to. Bonemerang OHKOs Magneton even at +0 Attack, bypassing its Sturdy; Heliolisk, instead, should be OHKOed by Dig, which is only possible after one Swords Dance or more: its Grass Knot 2-3HKOs Marowak, and the possibility of Bonemerang failing would ruin the matchup. For an even quicker victory, Marowak can hold the Expert Belt, which enables it to 2HKO Emolga and OHKO Heliolisk without any Attack boosts.
Rival (Route 14): Meowstic has learned Psychic, which turns the matchup against it upside down. Psychic can 3HKO, and Bonemerang is not guaranteed to OHKO after a single Swords Dance; Marowak may attempt to face Meowstic either way, but is likely to need a switch and a heal if it does. Marowak's health is best saved for Absol, which it can OHKO with Bonemerang after a Swords Dance. Delphox is OHKOed regardless of setups; Greninja and Chesnaught 2HKO Marowak unless its Swords Dance setup has reached a +4 or higher, but it is nigh impossible for Marowak to get there in the context of this battle, so it should just avoid the last two starters.
Gym #6 - Valerie (Laverre City, Fairy-type): While Mawile can be OHKOed with Bonemerang, it is better to use it for setups instead, to defeat Valerie's other two Pokémon more easily. At +2 Attack, Marowak can OHKO Mr. Mime too, but needs +4 for Sylveon; it is, however, recommended to go all the way to +6 with Swords Dance, because Mr. Mime is faster than Marowak and might set up Reflect on the first turn. If it does, a +6 in Attack will still guarantee the OHKO against it, and a 2HKO against Sylveon, from which Marowak can survive a Dazzling Gleam.
Celosia and Bryony (Poké Ball Factory, tag battle with rival): Liepard is 2HKOed by Bonemerang and Manectric is OHKOed, if Marowak holds the Expert Belt. Neither of them deals much damage to Marowak.
Mable (Frost Cavern): Expert Belt Bonemerang says hi.
Rival (Anistar City): Meowstic once again prevents Marowak from setting up, unless a single Swords Dance is enough to OHKO it with Bonemerang, an unlikely scenario. Absol has also become significantly more threatening, acquiring Night Slash; backed by a Swords Dance, it can OHKO with a critical hit, and Absol's ability is Super Luck. With a prior Swords Dance, however, Marowak can OHKO with Bonemerang. Jolteon and Flareon are OHKOed even just by Dig without any setups; Delphox is, as well, but only by Bonemerang (or by Dig after a Swords Dance) and Marowak needs well over half its health to survive a Psychic from it. Vaporeon, Chesnaught and Greninja are no-gos: their STABs are strong, and Marowak simply lacks the necessary space to set up Swords Dances and stand up to them.
Gym #7 - Olympia (Anistar City, Psychic-type): The battle against Olympia is difficult for Marowak. While it can 3HKO with Rock Tomb outspeeding on the very last turn, or 2HKO but with a smaller chance (and only with the Expert Belt), Sigilyph's Reflect instantly nullifies Marowak's efforts and prevents it from scoring the kill. Only a Thick Club Marowak can win against Sigilyph for sure. Slowking is 3HKOed by Bonemerang instead, and its Psychic also 3HKOs, but Slowking is one of the rare Pokémon that Marowak outspeeds. Then, Meowstic once again beats Marowak with Psychic, and without setup chances, since Meowstic's Calm Mind becomes threatening sooner than Marowak's Swords Dance.
Lysandre battle #1 (Lysandre Labs): Mienfoo can be 2HKOed with Bonemerang or Aerial Ace + Expert Belt, but Mienfoo will also nearly one-shot with High Jump Kick if it uses Swords Dance itself; Marowak will not go down in one hit, but it will need healing afterwards. Murkrow is the reason why Marowak should not attempt a Swords Dance against Lysandre's team: its Foul Play becomes a 2HKO at +2; instead, Marowak should 2HKO it with Rock Tomb or Return. Bonemerang OHKOs Pyroar most of the time with the Expert Belt attached, though if not, Marowak will need to switch out after taking a Fire Blast. Even a Thick Club Marowak has no chances against Gyarados, though, since its Aqua Tail is a potential OHKO.
Aliana (Lysandre Labs): Marowak's matchup is ideal if it uses Power-Up Punch once against Mightyena, and then finishes it off with Bonemerang, then 2HKOing Druddigon with Bonemerang as well. Swords Dance is not viable in this fight, as Mightyena can prevent it with Taunt.
Celosia (Lysandre Labs): Bonemerang OHKOs Manectric and 2HKOs Drapion, unless Acupressure boosts its Defense.
Bryony (Lysandre Labs): Marowak can easily win this fight by 2HKOing Liepard with Power-Up Punch and then OHKOing Bisharp with Bonemerang. Liepard also knows Taunt, so Swords Dance should not be used.
Mable (Lysandre Labs): Marowak can beat either Houndoom with Bonemerang, or Weavile with Power-Up Punch, a 2HKO. Weavile's Ice Shard is only a 3HKO, unlike Houndoom's Flamethrower, so Marowak can theoretically be healed every other turn to finish the fight, unless nuzlocke rules forbid this.
Xerosic (Lysandre Labs): Crobat is OHKOed by Rock Tomb after two Swords Dances and with the Expert Belt attached, but Marowak should be healed before beating it, if it must also face Malamar; Malamar's first Superpower, before Contrary factors into the equation, can 3HKO Marowak. Bonemerang, instead, is highly likely to OHKO Malamar at +4 in Attack.
Lysandre battle #2 (Team Flare Secret HQ): Marowak cannot beat the now-evolved Mienshao, which 2HKOs with High Jump Kick; only a gamble with Protect and Bonemerang may do it in, but if Mienshao uses Swords Dance instead, the situation will only worsen. Honchkrow, instead, can be 2-3HKOed safely with Rock Tomb, and Pyroar is OHKOed by Bonemerang. Gyarados can still OHKO with Aqua Tail, and must be left alone.
Xerneas (Team Flare Secret HQ, X only): No. Marowak's very average special bulk is not enough to take repeated Moonblasts.
Yveltal (Team Flare Secret HQ, Y only): Again, no. Yveltal 2HKOs almost for certain with Oblivion Wing, and the damage Marowak deals is far too low in comparison.
Lysandre battle #3 (Team Flare Secret HQ): Mienshao beats Marowak with High Jump Kick, though an Expert Belt Marowak can still 2HKO Honchkrow with Rock Tomb and OHKO Pyroar with Bonemerang. Mega Gyarados now has a guaranteed ticket to victory with Aqua Tail, and Marowak should come nowhere near it.
Professor Sycamore (Couriway Town): Venusaur OHKOs with Petal Dance, and is thus unapproachable for Marowak; same for Blastoise, which 2HKOs with Aqua Tail. Charizard is OHKOed by Rock Slide and the Expert Belt instead.
Shauna (Route 19): Delcatty is really easy for Marowak, and can be used as Swords Dance fodder; at +4 or higher in Attack, Bonemerang OHKOs both Delcatty and Goodra. Unless Marowak's Attack is much lower than average, Goodra will not survive a Dig, either. Marowak needs to be healed before facing Delphox, however, as it can take a Psychic only if its health is well above half; Chesnaught and Greninja are not Marowak's job, they both 2HKO it.
Tierno (Route 19): Talonflame is nuked with Rock Slide, but Roserade's Petal Dance and Crawdaunt's Crabhammer nuke Marowak in turn.
Trevor (Route 19): The relatively harmless Raichu can be used for Swords Dance setups before being offed. At +4 in Attack and with the Expert Belt, Marowak OHKOs Raichu and Florges with Bonemerang, and Aerodactyl with Rock Slide.
Gym #8 - Wulfric (Snowbelle City, Ice-type): Abomasnow wins against Marowak, but a healthy Marowak can live an Ice Beam from Cryogonal unless its nature is detrimental to Special Defense, and respond with a Rock Slide OHKO. Avalugg must not be attempted, as it one-shots Marowak with Avalanche.
Rival (Victory Road): Meowstic 3HKOs Marowak with Psychic, and Marowak has no hope of winning whatsoever, short of holding the Thick Club. Jolteon and Flareon are OHKOed by Earthquake, but Vaporeon can nearly OHKO with Muddy Water, and must not be fought. Altaria can be beaten with a Swords Dance and Stone Edge, though the move has a high chance of missing; Rock Slide will only OHKO a third of the time even with the Expert Belt attached, and Altaria can 3-4HKO with Dragon Pulse, so the matchup is entirely dependent on miss rates and damage variation. Alternatively, Marowak may be temporarily taught Ice Beam to 2HKO Altaria safely. Absol can be 2HKOed with Earthquake, but without any prior Swords Dance setups Marowak is risking it, because its Night Slash can OHKO after one of Absol's Swords Dances if the Super Luck support does net the critical hit. Marowak is also no longer guaranteed to OHKO Delphox with Earthquake and the Expert Belt, and now more than ever it needs to avoid both Chesnaught and Greninja.
Elite Four Wikstrom (Pokémon League, Ironworks Chamber, Steel-type): At long last, Marowak gets a very good matchup against an important figure in the Kalosian scene. Earthquake OHKOs Klefki with the Expert Belt attached, and Flamethrower 1-2HKOs Scizor. Bonemerang will also OHKO Probopass, breaking through its Sturdy. Marowak can also take an Aegislash-Blade's Shadow Claw (or two) and then OHKO with Earthquake, or 2HKO Aegislash-Shield with the same move; Swords Dance would make it an OHKO, but Marowak does not need it, because Aegislash is faster and Marowak is thus guaranteed to strike it in its Blade form.
Elite Four Malva (Pokémon League, Blazing Chamber, Fire-type): Expert Belt Marowak has a certain OHKO ahead against Pyroar, Chandelure and Talonflame, the first two with Earthquake and the last with Rock Slide. It cannot, however, take three hits in succession, so it will need to be healed during the battle. Earthquake also 2HKOs Torkoal, but its moves are the weakest of all, so Marowak will not have problems against it.
Elite Four Drasna (Pokémon League, Dragonmark Chamber, Dragon-type): Earthquake OHKOs Dragalge if the Expert Belt is being held; Druddigon is 3HKOed instead. Ice Beam can 2HKO Altaria and Noivern, both of which can only 3HKO Marowak at best. Marowak will need some healing, but it can solo Drasna.
Elite Four Siebold (Pokémon League, Flood Chamber, Water-type): No. All of Siebold's Pokémon 2HKO Marowak, Clawitzer may even net the OHKO, and Marowak's moves are not good enough against them.
Champion Diantha (Pokémon League, Radiant Chamber): Hawlucha can be 2HKOed with Ice Beam if Marowak holds the Expert Belt; this is the fastest way for Marowak to KO it. Tyrantrum can be 2HKOed with Earthquake, or OHKOed after a Swords Dance; the setup is usually pointless, however, as Aurorus' Blizzard is a guaranteed OHKO and will force Marowak to switch out. Gourgeist, Goodra and Mega Gardevoir can all 2HKO Marowak with their strongest moves, and must be avoided as well.
Post-Game: Common sense is the key, but in general terms, Marowak is still a decent Pokémon to use, against anything that cannot hit it with super effective moves.
Cubone's initial moveset is Tail Whip, Bone Club, Headbutt and Leer. For a basic Pokémon with very good Defense, this is actually very workable, apart from Bone Club's less-than-perfect accuracy; the good base power compensates for it, though, and Headbutt serves well against birds and whatever resists Ground. It then learns Focus Energy at level 17, but it isn't really worth keeping. Bonemerang, instead, is a heaven-sent coming straight at level 21, with the same base power as Earthquake; even with Cubone's not so high Attack, the move will wreck face in unthinkable ways. It even breaks through Sturdy, due to being a two-hit move. Rage comes at 23, but is plain dangerous and not worth using; False Swipe comes at 27, and can be worth considering to catch wild Pokémon more easily, though at that point in the game, the False Swipe TM is likely already available. After evolving, Marowak learns Thrash at 33, which is not as dangerous as Rage, but still definitely not the right move to use in most cases, as Marowak is slow and getting locked for two or three turns against the wrong opponent could spell death for it. Fling, at 37, is the embodiment of uselessness and should not even be considered. Bone Rush comes at 43, but Bonemerang should be kept over it, because Bone Rush is unreliable and its average damage is lesser than Bonemerang's, even though it does have a small chance of dealing more. Then, there is the not so useful Endeavor at 49, and at 53, the line learns Double-Edge. While not great coverage-wise, it can be a good move to have for Rock Head specimens, thanks to the lack of recoil; Lightning Rod carriers should take Return over it. Lastly, at 59, Retaliate becomes available, but it's long since past the point of usefulness at this stage.
The TM pool of this line is very good. In the late game, Earthquake is a must have, naturally, but Bonemerang will work the same way (except with slightly lesser accuracy) until then. Rock coverage is also very useful in general, specifically in the form of either Rock Slide or Stone Edge, depending on whether the player's style favours power over accuracy, or the other way round. Rock Tomb is also available not long after Cubone is caught, and in this generation, it is actually a decent move that Cubone and Marowak should keep until they get access to its stronger equivalents. Marowak can also get a welcome upgrade to its otherwise very average Attack stat in the form of Power-Up Punch, or Brick Break for simple Fighting coverage; Swords Dance is also an option if the power-up must be faster, and/or if Marowak needs to set up against Ghost-types. Lastly, and perhaps surprisingly, Marowak's low Special Attack does not prevent it from being a relatively useful Ice Beam and Flamethrower user in some situations, specifically against the Pokémon that have a double weakness against Ice or Fire moves, and may or may not have good physical bulk, Scizor being a shining example.
Recommended moveset: Power-Up Punch / Swords Dance, Double-Edge (with Power-Up Punch and Rock Head) / Brick Break (with Swords Dance and/or Lightning Rod), Earthquake, Rock Slide
Steel-types: Marowak's Grass and Ice weaknesses are both excellently covered by any available Steel-type that does not have a secondary typing which compensates for the two relevant resistances. Marowak also easily deals with Fire-types, and can take on most Fighting- and Ground-types, the sworn enemies of Steel Pokémon.
Electric-types: If complemented by a Steel-type, Marowak's only weakness left will be Water; although Grass-types are common and usually do a better job at covering for Water than Electric-types do, Electric-types are advisable, since they do not share an Ice weakness with Marowak. A good Electric-type should preferably also be fast, to compensate for Marowak's slowness.
Fast hitters: All of Marowak's stats are anywhere from decent to good, save for Speed. Since it also has no way of boosting it to hit first, Marowak should always be complemented by at least one fast Pokémon on the team, preferably one without any shared weaknesses.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Cubone Line Ratings
What Nature do I want? Anything that lowers Speed or Special Attack, and boosts something else. The best are Adamant and Brave for offensive players, and Impish or Relaxed for defensive ones. Careful and Sassy are also good.
Which Ability do I want? Rock Head is ideal to run Double-Edge. Otherwise, Lightning Rod is also fine. Neither of them is particularly crucial to Marowak's usefulness.
At what point in the game should I be evolved? If possible, before the fight with Korrina in Geosenge Town. Her Lucario are very powerful, and Marowak is one of the few Pokémon that can handle them with ease.
How good is the Cubone line in a Nuzlocke? Its Speed and underwhelming damage output can be a problem in some occasions, but the very good bulk - decent even on the special side - compensate for its shortcomings. Marowak is a better-than-average Pokémon overall, works well as a shield for pesky Electric-types and possible bomb moves, and has the privilege of lacking double weaknesses, which set it apart from the likes of Golem. It may not be exceptional, but it is well-rounded and unlikely to disappoint or die unexpectedly.