Dwebble is available in both Black and White. It can be found in the Desert Resort (in deep sand) and on Route 18 (in regular grass). Crustle can also be found on Route 18 (in dark grass).
Dwebble is a very useful Pokemon for Nuzlockers. Its movepool is adequate for its needs, and it is a solid physical tank when fully evolved. Both of its abilities are excellent security blankets in battle, and it has access to one of, if not the outright best boosting move in the game: Shell Smash. However, it definitely needs help from teammates to have its greatest impact. Even after a Shell Smash boost, Crustle will be outsped by quite a few threats, several of which can do large amounts of damage to it. Further, its low Special Defense hampers its usefulness against some bosses. Despite these flaws, though, Crustle can be a force on the right team. Effective base 190 Attack after a boost is nothing to sneeze at, and it can be helpful to its team even when it isn't trying to sweep.
N (Nimbasa City): Dwebble can't do even neutral damage to Sandile without leveling for Bug Bite. With that move, though, it achieves a guaranteed 2HKO and doesn't take a whole lot of damage in return. It is, however, inadvisable to take on Scraggy. Even Bug Bite doesn't let Dwebble outdamage it, especially if Dwebble has faced down Sandile already. Smack Down is a nearly-guaranteed 2HKO against Darumaka, but its Fire Punch can take away up to half Dwebble's health in one blow without a critical hit, so keep your Dwebble shipshape if you want or need to use it there. Lastly, Sigilyph has two STAB special moves; that's a hard nope for Dwebble's puny base 35 Special Defense, and even if it wasn't Smack Down doesn't even come close to a 2HKO against Sigilyph's bulk.
Gym #4 - Elesa (Nimbasa City, Electric-type): Dwebble has base 35 Special Defense. You aren't seriously considering sending it out to fight a Gym Leader who spams Volt Switch, are you? Even Elesa's Emolga cleanly 2HKO all Dwebble but those in the highest few IV tiers, and it can only achieve a guaranteed 2HKO back with an extremely high Attack IV. Leave the poor hermit crab out of this.
Cheren (Route 5):
Gym #5 - Clay (Driftveil City, Ground-type):
Bianca (Driftveil City):
N (Chargestone Cave):
Gym #6 - Skyla (Mistralton City, Flying-type): A Crustle evolved as soon as possible takes an absolute maximum of 40% of its health from Swoobat's full-power Acrobatics; in return, Rock Slide is a strong 2HKO at worst. Dwebble fares worse in both respects, but will in the vast majority of cases still avoid the 2HKO from Acrobatics and two-shot with Rock Slide. Dwebble, on the other hand, shouldn't face Unfezant; Air Slash is a solid 2HKO, while Rock Slide is a shaky one, not even being guaranteed with a 19 or lower Attack IV. Crustle can succeed in this matchup, though, since Rock Slide 2HKOs even with a zero Attack IV and an Attack-lowering Nature. Swanna knows Bubblebeam, which will eviscerate Dwebble and bring Crustle down to 20% of its health or less from full. Crustle cannot OHKO back, so it's highly recommended that you bring another Pokemon to defeat Swanna. Note that this analysis assumes you haven't overleveled for Shell Smash; if you have, then the outlook gets a whole lot brighter. A Level 37 Crustle can easily set up on Swoobat and plow through Unfezant. It does run the risk of Swanna still being faster after a boost, though, depending on IVs.
Cheren (Route 7):
Gym #7 - Brycen (Icirrus City, Ice-type): Crustle is guaranteed to tank the initial attack from Brycen's Vanillish, either through Sturdy or through Shell Armor; a critical hit Mirror Shot is necessary for Vanillish to even approach a OHKO. If you use that turn to boost with Shell Smash, Crustle is also guaranteed to outrun Vanillish at +2 Speed and can OHKO with boosted Rock Slide. From there, you need to be a bit more cautious. Cryogonal will likely outspeed Crustle even after the Shell Smash boost and, if the Special Defense drop isn't fixed, can OHKO with critical Aurora Beam. Crustle can tank Cryogonal's Frost Breath from full health, though. Another potential worry is that Cryogonal might set up Reflect; this will make your life much harder and might force you to abandon the sweep attempt. Crustle can OHKO Cryogonal itself through Reflect just fine, but Beartic presents problems. Reflect turns 2HKOs into 3HKOs and OHKOs into 2HKOs (depending on your Crustle's IVs and Nature), and Beartic has Brine. A critical hit or a hit against a Crustle below half health spells certain doom, especially if you don't fix the Special Defense drop. If Beartic comes onto the field with Reflect up, it's best to just abandon ship and try to win another way; if there is no Reflect, you're more likely than not to OHKO, with Rock Slide, and you're guaranteed to outspeed.
Bianca (Route 8):
Gym #8 - Iris/Drayden (Opelucid City, Dragon-type):
Cheren (Route 10):
Elite Four Shauntal (Pokémon League, Ghost-type):
Elite Four Marshal (Pokémon League, Fighting-type):
Elite Four Grimsley (Pokémon League, Dark-type):
Elite Four Caitlin (Pokémon League, Psychic-type): While it may be tempting to exploit Crustle's super-effective Bug STAB in this fight, its mediocre special bulk makes it hard for Crustle to be effective. Even with a perfect IV and a Special Defense boosting Nature (not to mention the assumption that you leveled for N and Ghetsis), her lead Reuniclus's Psychic has a slight chance to 2HKO Crustle. This makes it near impossible to set up Shell Smash safely, although Crustle does outspeed under those circumstances unless it has a very low Speed IV and a Speed-reducing Nature and 2HKO with X-Scissor. Musharna's Psychic is also a somewhat likely 2HKO, while X-Scissor is still a guaranteed 2HKO (not factoring in any potential Charge Beam boosts or Reflect setup on Musharna's part). Rock Slide is a strong 2HKO against Sigilyph, and its Psychic is a somewhat likely 2HKO, as before; however, you won't be outspeeding this one. And finally, Gothitelle is in a similar boat to the first two, but without the potential for Crustle to outspeed. As you can see, this fight is full of more or less 50/50 situations for Crustle; it's only recommended to use Shell Armor Crustle against Caitlin, to prevent unfortunate crit OHKOs. Better yet, find a more secure route to victory if you can.
All Dwebble caught in the Desert Resort will have the moves Sand-Attack, Faint Attack, Smack Down, and Rock Polish. Of these, only Faint Attack and Smack Down are immediately useful; Dwebble is incredibly frail on the special side and doesn't really want to take the risk of getting hit by special attacks while boosting or debuffing the opponent (extremely specific circumstances excepted). Bug Bite is learned at Level 23 and serves as a very solid secondary STAB; it is followed at Level 24 by Stealth Rock, which can provide useful chip damage in boss fights but isn't all that useful in run-of-the-mill Trainer fights. Level 29 brings along Rock Slide, which is a very good move and will probably be your primary attacking option through the end of the game. The next level-up move is Slash, but that move isn't exceptionally useful except as crit-fishing filler. If you delay Dwebble's evolution by one level, you can get X-Scissor early (at Level 35) for upgraded Bug STAB; if you choose not to delay evolution, it comes at Level 38. The piece de resistance of the line's level-up moves is Shell Smash, which comes at Level 37 for Dwebble and Level 43 for Crustle. Flail (Level 50) is entirely worthless on non-Sturdy Crustle and situational at best on Sturdy individuals. Rock Wrecker (Level 55) is essentially a physical Rock-type Hyper Beam, which can be useful in certain situations but is not recommended for most battles due to the recharge turn.
As far as TMs go, Crustle gets only a few truly worthwhile options. Shadow Claw (found in Celestial Tower) could be a decent matchup-specific coverage move, while Dig (Route 4) can give it the excellent Rock/Ground coverage combination. Aerial Ace (Mistralton City) is really weak and doesn't really add any coverage Sadly, all of its other good options, like Earthquake and Stone Edge, are stuck in the postgame. You can also reteach X-Scissor (Route 7) and Rock Slide (Mistralton Cave) via TM if for some reason you remove them (hint: you shouldn't).
Recommended Moveset: Shell Smash/Stealth Rock, Rock Slide, X-Scissor, Dig/Shadow Claw/Aerial Ace
Counters to Water types: Crustle is weak to Water type moves and doesn't learn any moves that really help it cover that weakness. Therefore, it is well complemented by Pokemon that can tank Water type hits and remove Water-type Pokemon from the field efficiently.
Good examples of this kind of Pokemon include: Serperior, Zebstrika, Maractus, Sawsbuck, Galvantula, Ferrothorn, Eelektross, Haxorus, Druddigon
Specially bulky Pokemon: Crustle's bulk is physically focused, and it sometimes struggles to stand up to strong neutral special attacks. Therefore, it should be paired with Pokemon that are well equipped to take those attacks.
Good examples of this kind of Pokemon include: Musharna, Audino, Scrafty, Gothitelle, Reuniclus, Escavalier, Amoonguss, Jellicent, Ferrothorn, Mandibuzz
Fast Sweepers: Even after a Shell Smash boost, Crustle is liable to be outrun by Unova's faster threats. Therefore, pairing it with a fast, powerful Pokemon that can remove those threats from the equation is a good idea.
Good examples of this kind of Pokemon include: Zebstrika, Scolipede, Archeops, Galvantula, Haxorus, Mienshao, Durant
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.
Dwebble Line Ratings
What Nature do I want? Anything that decreases Special Attack is a good option, but the stats Crustle wants most to boost are Attack and Defense. So, Adamant and Impish are your ideal outcomes. Speed-decreasing natures are probably second best, since any decrease in Crustle's Special Defense narrows its margin for error.
Which Ability do I want? Both Sturdy and Shell Armor are useful. Sturdy forms a powerful combination with Shell Smash, and if you catch a Sturdy Dwebble you should strongly consider making it into a dedicated attacker. Shell Armor's critical-hit-neutralizing effect is an amazing boon for tankier variants, and helps Dwebble get the most out of Stealth Rock. So, your ideal Ability changes based on what you want out of Dwebble.
At what point in the game should I be evolved? This sort of depends on how long you're willing to wait for early access to X-Scissor and Shell Smash. If you aren't interested in getting those moves early, then you should be evolved by Skyla. If you only want X-Scissor early, then you can probably still evolve before facing her; if you wait for both moves, then you should definitely be evolved by the time you enter Icirrus City. Note also that the longer you wait to evolve, the farther Dwebble falls behind its teammates in terms of pulling its own weight.
How good is the Dwebble line in a Nuzlocke? It's solid, but not a team-carrier in its own right. Its physical bulk is useful against a variety of opponents, and its STAB types are super effective against several key boss Pokemon; Shell Smash turns it into an extremely potent offensive force to supplement those good matchups. However, it struggles against special attackers and has to spend extra time in its not-fully-evolved form to make maximum use of its better moves.