Moves
Stealth Rock provides the utility of wearing down each foe that switches in. Discharge is a STAB move that has a high chance to paralyze, allowing Stunfisk to cripple opposing Pokemon and sweepers. Earth Power is a good STAB move that provides coverage on opposing Electric-types, such as opposing Stunfisk. Toxic is a good move to cripple opposing walls that are not hit hard by Stunfisk's other attacks, and it has good synergy with Earth Power, as Earth Power threatens Poison- and Steel-type Pokemon that attempt to switch into Toxic. Yawn is another option if Stunfisk wants to force switches or immobilize the opponent. Protect can also be run to scout foes and gain more Leftovers recovery, and Sludge Wave can hit Leafeon on the switch in and break Rotom-F's Substitute.
Set Details
252 HP EVs add to Stunfisk's overall bulk, allowing it to sustain more hits and spread more status. 252 Defense EVs with a Bold nature maximize Stunfisk's Defense stat, allowing it to take any physical hit well and possibly paralyze the opponent with Static. However, a specially defensive spread can be used to take hits from special attackers. The aforementioned Static is the ability, as it has a high chance to paralyze the opponent if hit on contact. Leftovers is used for passive recovery, although Passho Berry can also be run to check offensive Water-types such as Floatzel.
Usage Tips
Stunfisk should stick around throughout the match as a pivot and spread status with Discharge and Toxic to cripple most of the opposing team. Stealth Rock is best used after Stunfisk has already induced status or if it forces a switch due to type advantage. With its good Defense and Static, Stunfisk does a great job of keeping physical attackers, such as Dodrio, Purugly, and Pawniard, in check. This means that Stunfisk can pivot into physical attacks to try to activate Static. U-turn, in particular, is a great attack to pivot into, as users of the move are normally reliant on being quick. That being said, Stunfisk should not pivot recklessly into attacks that are powerful, as they can easily wear it down. If running Passho Berry, Stunfisk should be used to lure in and KO Water-type attackers such as Simipour and Swanna. Not revealing its lack of Leftovers can help lure these threats in, as, once it is clear Stunfisk is not carrying Leftovers, they are more likely to be wary of Passho Berry.
Team Options
Stunfisk's paralysis support is greatly appreciated on balanced teams by Pokemon that are relatively slow albeit powerful, such as Bouffalant and Regice. These two Pokemon also can cover Grass-types for Stunfisk. Roselia and other hazard setters can set their hazards in combination with Stunfisk's Stealth Rock to apply extreme pressure to the opponent. Grass-types such as Roselia can keep Water- and Ground-type Pokemon at bay, making them good partners. Clerics such as Audino and Grumpig are good partners, as they rid Stunfisk of poison and burns, and Audino can provide Wish support as well. Spinblockers such as Gourgeist can also keep Stealth Rock on the field, as well as other hazards. If Defog is a threat, Pawniard is a good partner, as the evasion drop from Defog activates Defiant and boosts Pawniard's Attack by two stages. Swanna and Dodrio also have great synergy with Stunfisk, while Stunfisk can block Volt Switch for them, which they both despise.
Other Options
Choice Specs is a viable option on Stunfisk, as it finds a lot of opportunities to switch in and can do surprisingly good damage. Rest can be run in the last slot either with a Chesto Berry as the item or with cleric support, but it generally won't be worth the loss of Leftovers recovery or reliance on clerics. Foul Play seems like a good option to hit physical setup sweepers, but it is rather weak without a STAB boost, and Stunfisk would much rather use a status-inducing move. Scald has a chance to burn but is usually inferior to Discharge, as it has worse coverage, doesn't get a STAB boost, and doesn't always cause a burn, whereas Discharge is also backed up by Static. Thunderbolt is better on stall teams that would rather have Stunfisk poison foes with Toxic, as well as having a higher damage output.