Shedinja isn't overpowered but it would be pretty boring to fight. Like evasion, more annoying than powerful. It's already been half-freed for a while so I don't mind whatever happens. Speaking of Shedinja, this post is about yet another counter.
Here's a gimmick I was playing around with and had a good time revisiting today. Just needed a /ds, but I ended up getting my metronome fix.
Gulpone (Cramorant-Gorging) @ Rowap Berry
Ability: Gulp Missile
EVs: 252 HP / 252 Atk / 252 Def / 252 SpA / 252 SpD / 252 Spe
Relaxed Nature
- Metronome
Gultone (Cramorant-Gorging) @ Rowap Berry
Ability: Gulp Missile
Shiny: Yes
Happiness: 160
EVs: 252 HP / 252 Atk / 252 Def / 252 SpA / 252 SpD / 252 Spe
Relaxed Nature
- Metronome
The team features the usually unusable Cramorant-Gorging. Gulp Missile will activate after any hit on Cramorant and deal 25% damage + paralyze thanks to its Gorging form (-1 Def with Gulping). It doesn't sound like much alone, and being locked to Cramorant is unfortunate. But Gulp Missile is one of the few harming abilities out there, and it's the most reliable of them. I would really even go as far to say that this is a hyper offense team. Gulp Missile puts pressure from the word go, and that can be enough to bridge the stat gap and win the most unlikely of matchups. On average I've seen it do more damage than the activating hit, though strong moves still hit hard. The speed nerf from paralysis helps with 2v1 situations, and Cramorant actually needs teamwork to stand a chance. Relaxed nature patches up Cramorant's weak defence, could focus the superior SpD but I like the balance. Speed doesn't matter too much, especially after the opponent is paralyzed.
The Rowap stacks the damage even further, so if an enemy rolls Swift or something they get hit for 25% + 25% + 12.5% + 12.5% = 75% damage in 1 shot. And that's not even on your turn. Even with just a single target, the opponent falls to ~63% health. Jaboca could probably work too, but the bird's special defence is much better and I feel like most spread moves are special. Shiny Cramorant gives you a funny minisprite on the field.
Weaknesses: Gulp Missile and Rowap Berry can teamkill. The strategy is a one-hit wonder; once the fish is spit it won't come back. There's a slim chance to reactivate it by using Surf or Dive. Some matchups can be bad. Electric types can't get paralyzed, and the -Defense form may not do so well against Defiant. Flower Veilers and Magic Guarders dodge half the effect. Having 2 paralyzers may not work out if one mon hits both, but this team is more for embracing the meme.
It's a hit or miss strat, but I guess it doesn't actually 'miss'. Gulp Missile will start you off running on a strong start, but then you still have to play the rest of the game, and the momentum is all Metronome. Moves and kills are still necessary for the win, but the damage threshold is a bit easier to reach and paralysis allows even more room for error. Endgame can be a struggle to win in, especially against defensive mons like Sableye. I think that the best scenario for Cramorant is a single target focusing all the damage into itself, and moving into a 2v1 early in the match.
To be honest I believe Gulp Missile is actually interesting and balanced for metronome battles. This ability has a unique gameplan unlike most harming abilities would have (letting passive damage add up throughout). Also Cramorant sucks. Water/Flying's resists are decent, but the Electric weakness is hard to deal with. Stats are nothing special, but that's a good thing. Imagine heracross with this. At the least, this team will likely get 1 kill. Even if you get OHKO'd Cramorant still shoots its shot, so it technically has the least worst case performance in the meta.
EDIT: Haven't played this scenario but I believe this should do well against Imposter in theory. -25% from 714 hp is a nice chunk to relieve some of the strain, and I believe that transformed mons can't use species specific abilities.