Your best bet at getting this achievement is playing as the Plants in the Graveyards VS Gardens game-type.
1) Select the "Pea-Shooter"
2) Abuse the "Bean-Bomb" ability whenever it's available (especially when zombies are attacking your garden)
3) Don't wait for your "Bean-Bomb" to recover.. Once used, just run out willy nilly and attack anything until you die.. The ability will recover once you respawn.
4)
IMPORTANT.. Killing non-human zombies counts towards your vanquish kills at the end of the game, so kill everything.
5) Try not to get pissed off would be my best advice.. there's a lot of good players and you won't get it every game.. Just enjoy the game and it'll eventually come.
This is a great game