I've just thought of an excellent idea for a revenge killer, it may sound a bit gimmicky but here me out:
Ditto @ choice scarf
Now what happens is the opponent KOs one of my pokemon and I use the free switch to send in ditto who immediately transforms into the opponent because of eccentric copying all there stats (except hp hence the EVs)[I]and[I] stat changes meaning that unless they have a choice scarf (which it doesn't copy) then I am guaranteed to out speed (because of my choice scarf) and will almost always KO them with there own moves, which is not only a very effective strategy but also has lol factor. After this I am safe to switch away as they send in their counter and repeat the process later on in the game. I think with it's new ability ditto will no longer be confined to the daycare centre for breeding and possibly even make it into OU!