Due to the unique way the Legends: Z-A Battle Club works, Stunfisk has been a genuinely decent choice due to its small size and the way it lies flat on the ground; its coloration also blends in with some of the environments. Being hard to spot/target helps cover for Stunfisk's mediocre defences, and the way that speed works in this game means it isn't crippled by its slowness. This means you can run a pretty straightforward offensive set without resorting to gimmicks.
Stunfisk @ Life Orb
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
- Thunderbolt
- Charge
- Earth Power
- Sludge Bomb / Protect
If possible, use an XS size Stunfisk. Thunderbolt + Charge is the primary STAB, being especially good for Skarmory and Gyarados, and Earth Power is STAB for Metagross and Fire-types like Chandelure and Delphox. You can run Sludge Bomb to use on Xerneas but it isn't all that effective so it might be better to swap for Protect.
Stunfisk is going to be a glass cannon who can't take hits from mostly anything, so play accordingly. Stealth and evasion are your main strategies here. You only get two teammates in this format but I recommend having at least one good option against Xerneas, such as Metagross, Skarmory (watch out for Thunderbolt), or Chandelure.
The meta for this format will change over time but generally Stunfisk is probably going to remain usable but niche. It's not a top pick, but you can be successful if you're committed to it.
Not the same set as mine, but you might enjoy this video of a player using Stunfisk.
Otherwise, in some formats, Stunfisk can possibly work as part of a sand team, but it would be better as a more defensive role—probably not a great sweeper like you're looking for. It's difficult to pull off a sweeper role in most classic formats when you have terrible speed and your defenses aren't spectacular.
