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My vote goes to NOVED. Mega Heracross can kill both of these mons quickly enough so that a toxic has no effect, and takes hits (even psychic STAB from slowbro) well enough to kill both Chansey and Slowbro without ever needing to switch out/getting KOd.
I usually run timid myself but found that modest picked up more damage, however that was when I was experimenting with non LO sets (focus sash mainly). Also the HP rock tips were helpful. Thanks for the constructive criticism!
Serperior @ Life Orb
Ability: Contrary
Nature: Modest
Level: 50
EVs: 4 HP/252 SpA/252 Spd
-Leaf Storm
-HP Rock
-Protect
-Giga Drain
This is a pretty obvious Serperior set. Leaf Storm+ contrary puts Serperior at +2, meaning it can OHKO Charizard Y with HP Rock and can already OHKO Hippowdon...