the easiest way, and the way i got it, is to go into living dead playlist, it's on every couple of weeks or so - bungie picks them out at random out of rocket brawl, grifball, this and that might be it not sure...
Anyway, i got this on the orbital map, but it was quite hard. you have to 'camp' at the top of your starting point as a human and kill them at top using grenades or preferably your shotgun.
one map to definitly NOT try it on is the dead docks one, although you can try for 'delicious brains' achievement, as the zombies are invisible. although you can try it on these following maps, my method is advisable.
the maps you can get this on are:
-Dead Docks (Longshore)
-Spooky House (Sandbox)
remember, you have to kill 2 zombies in 1 GAME. it does not have to be in 1 round, just 1 game.