This is a pretty good Whimsicott Set
Whimsicott @ Leftovers
Nature: Jolly/Timid
Ability: Prankster
252 Spe/ 252 Def
Substitute
Leech Seed
Protect
Endeavor
This set plays like a typical whimsicott set, with a catch. This set allows you to use whimsicotts awesome ability and low hp to take out some pokemon with relative ease. You basically run down your own hp with substitute, and your opponents HP with leech seed, and then you Endeavor them and have Leech seed finish them off.
One thing: don't you thing leftovers kind of goes against that strategy? If your goal is to lower your HP, then why regain it with lefties? I'd recommend... I don't know... maybe Sash? Or you could do a Cottonee with Eviolite.