Halo 3

One of the easier achievements to get. It has to be in a ranked free for all playlist, so it must be done in Lone Wolves. You should usually get it after a playing a match or two there. If not though, devote a whole match to beating down/assassinating people and you should get it. If you're lucky enough to get a sword match, then this achievement is a walk in the park as all sword kills count as beat downs.

This achievement needs no boosting in order to get this achievement. All you have to do is get into a Ranked Free For match and devote the entire game to assassinating people(coming from behind people and pressing the melee button) or melee fighting(just spaming the melee button until you kill him/her) do this 5 times and you will get this achievement easy.