Ferrothorn is probable counter for Whimsicott. It resists Leech Seed because of its Grass typing, So Whimsicott can do nothing without it. Cloyster can do serious damage to it with Icicle Spear to counter the Substitute. Another counter, the best Counter for Whimsicott is Magic Bounce Espeon and Xatu because it resists everything it can dish out, All of its moves that you listed. The last counters are Grass types like Venusaur. Also Heatran because it also resists Leech Seed and can really harm it with Fire Blast. The last counter I can think of is a Pokemon with the Magic Guard ability like Alakazam. Even though It does not resist Taunt, is can resist Leech Seed and hit hard as well.
