(Post deleted, as Slowbro was already taken. Will update later today with Trevenant.)
Hard
Check
Trevenant
Trevenant @ Sitrus Berry
Ability: Harvest
EVs: 252 HP / 4 Atk / 252 Def
Impish Nature
- Protect
- Leech Seed
- Will-O-Wisp
- Horn Leech
Trevenant is a vastly underrated spinblocker in OU, functioning especially well against sand offense, a huge threat in the current meta. As one of the only two ghost-types able to reliably spinblock against Starmie and Excadrill, it has some great utility.
Sitrus Berry is key here, allowing Trevenant to ALWAYS avoid the 2HKO from Iron Head, even with sand and rocks up. That gives the ghost tree a chance to WoW, neutering Excadrill, and then stall with Protect until Harvest kicks in.
252+ Atk Life Orb Excadrill Iron Head vs. 252 HP / 252+ Def Trevenant: 165-195 (44.1 - 52.1%) -- 16% chance to 2HKO
So, it looks like this:
First turn: Trevenant switches in to any of Excadrill's moves, taking minimal damage from everything but Iron Head. Assuming the worst, a max damage Iron Head, Sitrus Berry will activate.
Second turn: With Sitrus last turn, Trevenant can even survive another max damage Iron Head. It uses WoW. Harvest has a 50% chance of recovering Sitrus for another 25% healing.
Third turn: If Harvest activated last round and WoW hit, then you should have enough HP to start hitting back with Horn Leech or Leech Seed to recover more HP. If not, Protect to stall for another round in hopes of activating Harvest.
It's a check and not a counter because of WoW's shaky accuracy, and because even burned, Exca can take you down if Harvest doesn't activate at all over three rounds and you switches into an Iron Head. But it'll be neutered for the rest of the match - a burned Exca is no real threat.