Beldum was only available through a Nintendo Network distribution running from the release of the games through January 14th, 2015. Its Original Trainer is Steven, it holds a Metagrossite, and it is always shiny.
Broken as Beldum, average as Metang, then broken again as Metagross: such is the odd fate of the region's first pseudo-legendary. As an event Pokémon, Metagross is unlikely to end up in a great many nuzlocke teams; only nuzlockers who have kept the Beldum untouched on their cartridges for a lengthy period of time will get a chance to savour its usefulness in battle. Metang is not easy to take to Metagross status, but after that, there is little that can stop it; it has largely overwhelmingly positive matchups, even against bosses that specialise in a type it has a weakness against. Metagross is so strong that it rarely even needs Mega Evolution to win a matchup.
Rival (Route 103): Beldum easily 2-3HKOs any starter with Zen Headbutt or (not for Torchic) Iron Head, as none of them have anything but a Normal move.
Gym #1 - Roxanne (Rustboro City, Rock-type): A level 9 Beldum with Iron Head 2-3HKOs Geodude, but takes far too long to weaken Nosepass. A higher-levelled Beldum has much better chances: at level 14, Iron Head KOs Nosepass faster than Nosepass can KO Beldum; however, Beldum may disobey or refrain from attacking. A possible alternative is using a level 9 Beldum and setting up Iron Defenses against Geodude: at +6, Iron Head deals more to Nosepass than Rock Tomb does to Beldum. Beldum must not level up after beating Geodude, otherwise the risk of disobedience will once again present itself.
Gym #2 - Brawly (Dewford Town, Fighting-type): Beldum's Zen Headbutt can 2HKO both Machop and Makuhita, even if they use Bulk Up twice. Beldum should not be holding an item in this battle: this way, Makuhita's Knock Off will not be powered up.
Rival (Route 110): Metang easily 2-3HKOs Shroomish with Iron Head, and Wailmer with Zen Headbutt; it can also 2HKO Slugma, though Slugma can chip away up to half of its health with Incinerate. Grovyle is also easily 2HKOed by Iron Head; Combusken is 1-2HKOed by Zen Headbutt, but its Flame Charge is roughly as powerful as Slugma's Incinerate and it will outspeed on the second turn, if not already on the first. Marshtomp should be avoided, it knows Mud Bomb.
Wally (Mauville City): IRON HEAD. METANG DESTROY.
Gym #3 - Wattson (Mauville City, Electric-type): It is difficult for Metang to participate in this battle, but it can do some work with Power-Up Punch. As it outspeeds Magnemite, it will chip away roughly half its health, putting it in KO range in the following turn; if it uses Thunder Wave, it is recommended that Metang be healed, because slowness paired with the possibility of not attacking will severely worsen its performance. If Magnemite uses Volt Switch to have Magneton come into the battlefield, how long Metang may stay depends on its health and bulk: Magneton's Volt Switch is usually a 3HKO, though close to 2HKO range for high rolls; Power-Up Punch KOs after three turns, if there is a prior Attack boost from fighting Magnemite. The 2HKO against Magneton is only possible starting at +3, so Metang is unlikely to defeat Magneton by itself, unless healed a few times. Voltorb can be 2HKOed by Zen Headbutt at no boosts, or OHKOed starting at +2.
Tabitha/Shelly (Meteor Falls, tag battle with rival): Metang is at odds with Tabitha's team; Mightyena and Numel can both deliver strong super effective attacks, and Numel's Magnitude 10 even has chances of OHKOing with a high damage roll. Although Metang would easily defeat Koffing, it should not be used in Omega Ruby. In Alpha Sapphire, after Mightyena is defeated, Metang can enter the battlefield and dispose of either Grimer with Zen Headbutt or Carvanha with Power-Up Punch, both of which are 2HKOs; however, Metang is unlikely to be able to defeat both, as Carvanha's Assurance also 2HKOs and Grimer's Mud Bomb deals considerable damage.
Tabitha/Shelly (Mt. Chimney): Zen Headbutt 2HKOs both Koffing and Grimer, with an OHKO chance against the latter. Metang loses to Numel due to the unpredictability of Magnitude, but if still in high health, it can 2HKO Carvanha with Power-Up Punch.
Maxie/Archie (Mt. Chimney): Mightyena's Swagger and Assurance are an extremely dangerous combination, which Metang should avoid at all costs; Golbat is easy to 2HKO with Zen Headbutt, though Metang should not use Power-Up Punch against it, as the Attack boosts can backfire if Golbat spams Confuse Ray. Camerupt and Sharpedo also need to be left alone, as they can both 2HKO Metang.
Gym #4 - Flannery (Lavaridge Town, Fire-type): Metang is entirely inadequate for this battle. It can 2HKO Numel with Zen Headbutt or Bulldoze, but nothing more.
Gym #5 - Norman (Petalburg City, Normal-type): Metang cannot beat either Slaking, as both have Feint Attack and the move 2-3HKOs. Vigoroth also knows the move, but only manages to 4HKO Metang with it, and Metang can 3HKO it with Iron Head. Metang theoretically beats Vigoroth even with three Power-Up Punches, but the setup is still insufficient to defeat the ace Slaking, and thus pointless.
Courtney/Matt (Southern Island, tag battle with Steven): Metang should not battle against Courtney, as Camerupt's Earth Power is highly likely to OHKO; if Camerupt has already been defeated, Metang can sub in for an easy 2HKO against Koffing with Zen Headbutt. Sharpedo no longer knows Assurance and can now be 2-3HKOed with Power-Up Punch, though its Swagger still represents a threat, due to its higher Speed; Grimer is 1-2HKOed by Zen Headbutt.
Tabitha/Shelly (Weather Institute): Camerupt's Earth Power is a likely OHKO against Metang, and should be avoided. Sharpedo may represent a threat with Swagger, though Metang can KO it with Power-Up Punch if it manages to break through confusion.
Rival (Route 119): Metang easily beats Wailmer and Shroomish with Zen Headbutt, 2-3HKOing them, and can 2HKO Slugma with Bulldoze after enduring a Flame Burst. Grovyle is also 2HKOed by either Zen Headbutt or Iron Head. In order to also beat Marshtomp or Combusken, however, Metang needs a prior setup; it can use Power-Up Punch to beat Shroomish instead, which will guarantee the Zen Headbutt OHKO at +1 or higher, and a boost of +2 will guarantee it even in the event Combusken uses Bulk Up; Marshtomp needs to be outsped and 2HKOed, which is only possible if Metang's Attack is at +2 or more.
Gym #6 - Winona (Fortree City, Flying-type): Though Winona's Swellow may prove insidious with a combination of Double Team and Endeavor, Metang will manage just fine if it uses Power-Up Punch at least twice, which will then allow to KO Swellow with either of its main STABs. At +2, Metang also 2HKOs Pelipper with Zen Headbutt; Skarmory is tougher to beat, but its moves are also weak against Metang, so a regular Power-Up Punch spam will defeat it. Metang should not come close to Altaria, which knows Earthquake.
Courtney/Matt (Mt. Pyre): Camerupt either kills Metang in one go with Earth Power or comes close, though Sharpedo has no STAB moves and can be 3HKOed with Power-Up Punch.
Rival (Lilycove City): Metang 3HKOs Swellow after two Power-Up Punches with either Zen Headbutt or Iron Head; it can 2HKO Breloom as well, but needs to use Psychic to do so, else Breloom might Counter it to death. It needs to leave Magcargo and Wailord alone, however; Magcargo has high Defense and cannot be KOed before it deals significant damage with Lava Plume, and Wailord might outspeed and deal almost the same amount of damage with a full-power Water Spout. Even the Eviolite is not enough to counter either threat. The same goes for Blaziken and Swampert, which outpower Metang with Blaze Kick and Mud Bomb. Metang can take on Sceptile instead, 3HKOing it with Zen Headbutt or Iron Head.
Courtney/Matt (Lilycove Hideout): Metang should avoid Camerupt, which 1-2HKOs with Earth Power (2HKOs even with the Eviolite), but can 3HKO Sharpedo with Power-Up Punch counting the Attack boosts in between.
Gym #7 - Tate and Liza (Mossdeep City, Psychic-type): Metagross OHKOs both Lunatone and Solrock with Meteor Mash. Lunatone is OHKOed even by Iron Head.
Maxie/Archie (Seafloor Cavern): Mightyena knows no Dark moves and can be OHKOed with Hammer Arm. Muk and Crobat are OHKOed by Zen Headbutt; Psychic has a small chance of OHKOing, as well, but is not guaranteed to, whereas Weezing is OHKOed by Psychic but not by Zen Headbutt. Either way, none of these Pokémon have moves that are threatening for Metagross, which either resists or is immune to all of them. Metagross also 2HKOs Mega Camerupt with Earthquake; Mega Camerupt has no STAB, so Metagross is not in danger. Mega Sharpedo can, however, OHKO with Crunch before Metagross does so with Hammer Arm, so the matchup is risky without using Mega Evolution. Mega Metagross, on the other hand, can withstand a Crunch from Mega Sharpedo at full health, and OHKO with Hammer Arm; it can also OHKO all the Poison-types with Zen Headbutt.
Primal Groudon (Cave of Origin, Omega Ruby only): No. Even Mega Metagross is OHKOed by Precipice Blades.
Primal Kyogre (Cave of Origin, Alpha Sapphire only): Again, no. Mega Metagross has small chances of surviving an Origin Pulse, regular Metagross has none.
Gym #8 - Wallace (Sootopolis City, Water-type): Metagross should profit of Luvdisc to set up with Hone Claws; at +3, Zen Headbutt OHKOs all of Wallace's Pokémon. However, Metagross should also be healed from confusion in between, as it will hit itself for greater damage if confusion kicks in. Agility or Rock Polish are advised, as Luvdisc is likely to outspeed without any Speed boosts.
Wally (Victory Road): Metagross can use Altaria to spam Hone Claws until it hits +3; at that point, Meteor Mash will OHKO all of Wally's team except Magneton, which is 2HKOed by Zen Headbutt or Earthquake (it has Sturdy). Metagross may also want one Agility or Rock Polish to outspeed Mega Gallade, which would otherwise take away over half its health with Close Combat, possibly OHKOing with a critical hit. Mega Metagross only requires a +2 to net the OHKOs, and will not be OHKOed by a critical Close Combat if its health is full.
Elite Four Sidney (Ever Grande City, Dark-type): Metagross can take two Crunches from Mightyena, use Hone Claws twice, and OHKO all of Sidney's team with Meteor Mash. Mega Metagross only requires one boost to do the same.
Elite Four Phoebe (Ever Grande City, Ghost-type): Metagross should aim to 2HKO Dusclops with Meteor Mash or Shadow Ball before it can use Curse; it is important for Metagross to not keep Curse status on as it battles, so it needs to be switched out if a Curse is active by the time Dusclops is defeated. Either of the two Banette is good to set up a Hone Claws, which then allows Metagross to OHKO both of them, as well as Sableye, with Meteor Mash; do not set up against Sableye, or its Foul Play will OHKO Metagross after just one Attack boost. It should be noted that Metagross may require healing against the female Banette, which knows Will-O-Wisp. Meteor Mash 2HKOs Dusknoir without boosts, but can OHKO at +2 or higher; Metagross can win even without boosts, as Dusknoir's Fire Punch deals relatively little damage. Mega Metagross can OHKO the two Banette and Sableye even without any Attack boosts.
Elite Four Glacia (Ever Grande City, Ice-type): Although Glalie does know Crunch, the move is fairly weak against Metagross' very solid Defense; Metagross can thus use the first turn to boost its Speed with Agility or Rock Polish, and then OHKO all except Walrein with Meteor Mash or even Iron Head. Walrein, instead, is 2-3HKOed by Meteor Mash, which is still fine for Metagross, as Surf is barely a 3-4HKO.
Elite Four Drake (Ever Grande City, Dragon-type): Altaria's moves barely tickle Metagross, giving it plenty of wiggle room. Metagross needs to use Agility or Rock Polish once to ensure outspeeding, and then boost its Attack to +3 with Hone Claws in order to OHKO all of Drake's Pokémon with Meteor Mash. Mega Metagross accomplishes the same with just +1 in Attack.
Champion Steven (Ever Grande City, Steel-type): Skarmory barely hurts Metagross, which can easily spam Hone Claws all the way up to +6 and use Agility or Rock Polish once. With this setup, Metagross can 2HKO Skarmory with Return, and OHKO the rest of the team with either Return (Claydol) or Earthquake (the other Pokémon). Aggron will survive a hit due to Sturdy, but Metagross will also survive Earthquake from Aggron if its health is above half. For Mega Metagross, reaching +4 is enough to attain the same matchup.
Post-Game: Psychic resistance, Dragon resistance, Flying resistance, and the capability to Mega Evolve. Metagross is likely one of the very few Pokémon that can hold up against even the scariest of the post-League threats, and come out victorious.
Beldum's initial moveset is Iron Defense, Hold Back, Zen Headbutt and Iron Head. This moveset is absolutely broken in the early game, as it provides Beldum with two of its best physical STABs as well as Iron Defense early on; its typing is also resistant to Normal, which guarantees its early game ventures to be a cakewalk. Beldum does not learn other moves by level. Metang learns Confusion and Metal Claw upon evolving, but Beldum already knew better alternatives to those moves, so they should be passed up. At level 23, Metang gets Pursuit, which adds to its coverage, but will generally be underpowered due to the in-game AI rarely switching out. Bullet Punch, at level 26, is a more interesting option, which grants Metang the first hit regardless of its Speed, barring other priority moves from faster foes. Miracle Eye becomes accessible at level 29, but is useless; Zen Headbutt is relearned at level 32, in the event Metang forgot it. Scary Face, at level 35, provides an unreliable solution to Metang's Speed problems, which are much better solved by the later Agility. Metang then gets Psychic at level 38; though it will generally deal less damage than Zen Headbutt, it is more accurate, and can work on specially-based sets. Agility, at level 41, is a good move to have, especially since a single usage will typically guarantee Metagross to outspeed any enemy. Metang gets Meteor Mash at level 44, shortly before evolving; while slightly more powerful than Iron Head, it is also less accurate, although the potential to boost Attack may be tempting for fully offensive sets not running Attack-boosting moves. Metagross gains access to Hammer Arm upon evolution, a great coverage move, but it does not go well with Agility. It will then attempt to relearn Iron Defense at level 52, and lastly get Hyper Beam at level 60, but the move is not good and runs off Metagross' lower offensive stats.
Metagross' TM pool is as good as it gets. In addition to its level-up STABs, which are great on their own, it can learn Earthquake and Rock Slide, to complement its coverage. In the early-to-mid-game, Bulldoze and Rock Tomb can be taught to Metang for short-term utility, as well. Brick Break, while available, is generally not as useful as the other moves, nor as powerful; Hammer Arm will work better. Metagross greatly benefits from having Attack-boosting moves, so non-Meteor Mash users can choose between Power-Up Punch and Hone Claws; Hone Claws also goes well with Meteor Mash itself, as a single usage boosts its accuracy to over 100%, and does the same with Zen Headbutt. Metagross' Special Attack is also good, so it can run special moves such as Psychic, Shadow Ball and Grass Knot, or opt for Psyshock to hit the enemy Defense. Multiple support moves are also available: Metagross gets access to both Reflect and Light Screen, as well as Sandstorm, and can relearn Speed-boosting moves flexibly with Rock Polish.
Physical: Hone Claws / Power-Up Punch, Zen Headbutt, Meteor Mash / Iron Head, Hammer Arm / Earthquake / Agility / Rock Polish
Special: Psychic, Shadow Ball, Grass Knot, Agility / Rock Polish
Support: Reflect, Light Screen, Agility / Rock Polish, Meteor Mash
Water-types: Only Water-types can cover Metagross against both Ground- and Fire-types, its former only weaknesses. Fortunately, Water-types are very common, and finding good ones to pair with Metagross is not difficult. Interestingly, Levitate users with a Fire resistance also perform this role well, but the only Pokémon with one such combination that does not share any weaknesses with Metagross is Flygon.
Dark- or Fairy-types: Dark-types can cover both Ghost and Dark defensively, though their STAB is also resisted by Dark; Fairy-types are neutral to Ghost, but can hit Dark-types for super effective damage. Both of these types sport Pokémon that synergise well with Metagross in a team; Dark-types should preferably also have another STAB, for better coverage against the enemy Dark moves.
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.
Beldum Line Ratings
What Nature do I want? Metagross is excellent with any given nature. Since Attack and Defense are its best (and most used) stats, it will likely benefit the most from natures such as Adamant, Impish, Naughty or Lax; although its Special Attack is more than usable, its physical options will often overshadow it. Its Speed is also a valuable resource, so natures that detract from it will generally make its performance a little worse until Rock Polish or Agility can be added to its moveset.
At what point in the game should I be evolved? Beldum should evolve before Wattson, and Metang should evolve before Tate and Liza. It is important to evolve Metang into Metagross exactly at level 45, so that it may learn Hammer Arm naturally.
How good is the Beldum line in a Nuzlocke? By Hoenn standards, Metagross is a late bloomer, despite attaining both of its evolutions ten levels earlier than the other pseudo-legendaries; Metang struggles in a number of matchups in the middle game, though it never really does poorly. Metagross is a beast, and can fight almost anything without risks; once fully evolved, its power is so great that it will often not even need Mega Evolution to win.