Moves
Nasty Plot allows Croagunk to boost its Special Attack and punch holes into opposing teams, and it gets many setup opportunities due to its myriad of resistances. Thief can be used over Nasty Plot, which reduces Croagunk's wallbreaking potential but gives it more utility, as it lets Croagunk remove defensive items from foes after Croagunk takes a Knock Off or has consumed its Berry Juice. Sludge Bomb is Croagunk's primary STAB move, 2HKOing most of the Pokemon in the metagame that don't resist it after a Nasty Plot boost. It also has a decent chance to poison foes, which can help wear them down to the point where Croagunk can pick them off with Vacuum Wave. Focus Blast is a much more powerful but less reliable STAB move, which Croagunk can use in situations where it needs to deal more damage, securing a good chance to OHKO Timburr after a Nasty Plot boost. It also has terrific coverage alongside Sludge Bomb. Shadow Ball is an option in the third slot to keep this Croagunk set from being walled by Ghost-types such as Honedge and Gastly, but it gives Croagunk worse overall coverage, as it then lacks a powerful Fighting-type STAB move. Vacuum Wave gives Croagunk a STAB priority move that it can use to threaten revenge killers that can take advantage of its low Speed. Most notably, it does significant damage to Diglett, which would otherwise trap and remove Croagunk, though Diglett must have taken some prior damage to be KOed even if Croagunk is at +2.
Set Details
The given EV spread coupled with a Modest nature maximizes Croagunk's Special Attack, giving it maximum wallbreaking potential. The remaining EVs give Croagunk optimal bulk alongside Eviolite by giving it even-numbered stats in both defenses, which allow it to take a myriad of hits while setting up, most notably avoiding the 2HKO from Choice Scarf Mienfoo's High Jump Kick with an Eviolite. A point is also invested into Speed, to allow Croagunk to outpace most defensive foes, specifically RestTalk Mudbray. An alternate EV spread of 116 Def / 188 SpA / 196 Spe can be used to give Croagunk enough Speed to reliably outspeed offensive Mudbray and slow Eviolite Mienfoo, as well as tie with most variants of Vullaby, at the cost of some bulk. While an Eviolite is generally used to give Croagunk maximum bulk, Berry Juice is also an option for an item, reducing Croagunk's bulk but allowing it to take more hits over the course of the match, thus giving it more opportunities to set up Nasty Plot. However, as Croagunk frequently is hit by Knock Off early-game, it often appreciates the initial bulk provided by Eviolite more than the HP boost provided by Berry Juice. Dry Skin is Croagunk's most useful ability because it gives it a pseudo-immunity to Water-type moves, allowing it to reliably check many Water-type Pokemon such as Corphish and Mareanie.
Usage Tips
Croagunk should be brought in to check Pokemon that can't deal significant damage to it, such as Timburr, Mienfoo, Corphish, and Chinchou. After coming in, Croagunk will usually force a foe out. Against balanced and bulky offensive teams, it is generally better to either switch Croagunk out or attack early-game, as if Croagunk sets up, it will often be forced out immediately by the foe that comes in. Against slower-paced teams, hyper offensive teams, and balanced and bulky offense teams that have been worn down significantly, Croagunk can use Nasty Plot to begin punching holes in the opponent's team. Although this Croagunk set can be used as a glue Pokemon and switch directly into attacks that threaten its team, keep in mind that the lack of any kind of recovery means that it will get worn down quite easily, so bringing Croagunk in without it taking a hit is ideal when possible. Keep in mind that even with Eviolite, Croagunk is not especially bulky, so avoid allowing it to take super effective or powerful neutral hits while it is still needed in a match.
Team Options
This Croagunk set works well on teams that appreciate its resistances but don't rely on it as the sole check to all of the Pokemon it can check, which gives it more leeway to set up. Entry hazard support from Pokemon such as Onix, Dwebble, and Omanyte is helpful due to the switches Croagunk forces and allows it to secure more KOs with Vacuum Wave. Pursuit support from the likes of Alolan Grimer and Pawniard is useful to trap Abra and Ghost-types, which force Croagunk out, and to wear down faster foes to the point where they can be picked off with Vacuum Wave. Ground-types are especially problematic for this set, so teammates that can switch in on Ground-types, most notably Snivy and Pumpkaboo, make good partners. Flying-types can also shut this set down, so checks to these Pokemon, such as Onix and Omanyte, fit well alongside Croagunk, and Croagunk can take on the Fighting- and Grass-types that threaten them. Knock Off support is useful to help Croagunk secure more KOs. Mienfoo and Timburr both provide this support well and synergize decently offensively with Croagunk too, as they are able to form an offensive Fighting-type core.