That Whimsicott can easily be defeated by a Pokemon that knows Taunt and is resistant to GRA moves.
Haxorus is a great counter because it is unexpected (Haxorus is weak to Whimsicott's FAI type) and can set-up [*Dragon Dance*].
![Haxorus][1]
Haxorus@Assault Vest (increases SP.DEF by 50% but Haxorus can't use status moves)
Ability: Mold Breaker
EVs: 252 ATK, 252 SPE, 4 DEF
Nature: Adamant (+ATK, -SP.ATK)
Characteristic: Likes to thrash about (+ATK, 31IVs)
Moveset:
Taunt - Prevents Whimsicott from setting up and forces a switch because giga drain isn't very effective against Haxorus allowing Haxorus to set-up with...
Dragon Dance - Allows Haxorus to set-up if Whimsicott switches or attacks with an ineffective giga drain so it can sweep.
Outrage - Extremely powerful STAB dragon move that can servilely damage opponents as well as covering Haxorus's weakness to DRA.
Iron Tail/Brick Break - Extremely powerful steel type move that covers Haxorus's weaknesses to ICE and FAI/powerful fighting move that is super effective against steel type Pokemon that dragon moves are weak to as well as covering Haxorus's weakness to ICE.
Mega-Gengar is a better counter in my view. Though it isn't as good at setting-up for a sweep it can pretty much guarantee the death of Whimsicott because of Shadow Tag, it's SP.ATK = 170, it has STAB, 4x super effective, poison moves and resists GRA.
![Mega-Gengar][2]
Mega-Gengar@Gengarite
Ability: Shadow Tag (Prevents Whimsicott from escaping)
EVs: 252 SP.ATK, 252 SPE, 4 SP.DEF
Nature: Modest (+SP.ATK, -ATK)
Characteristic: Mischievous (+SP.ATK, 31IVs)
Moveset:
Taunt - Prevents Whimsicott from setting up and so forces Whimsicott to use giga drain which isn't very effective as it can't switch because of Shadow Tag.
Shadow Ball - Powerful STAB move.
Sludge Bomb - Powerful STAB move that is 4x super effective against Whimsicott.
Energy Ball - Powerful move that gives Mega-Gengar type covergae.
__ OR __
Taunt - Prevents Whimsicott from setting up and so forces Whimsicott to use giga drain which isn't very effective as it can't switch because of Shadow Tag.
Toxic - Weakens the opponent every turn even if Gengar faints..
Hex - Extremely powerful STAB move when the target has a status problem (like poison)
Venoshock - Extremely powerful STAB move when the target is poisoned that is 4x super effective against Whimsicott.
In fact, Haxorus and Mega-Gengar would work well on a team. Both have different roles, stat distributions, types and moves.
[1]: http://www.smogon.com/download/sprites/bw/642.png
[2]: http://www.smogon.com/media/sprites/xy/front/gengar-mega.png