Just as a more recent solution for anyone still going for this =]
Any kills count, you do not need to be the host for the kills to count, I was the host in a session and everyone gained their achievement!
Any kills gained in a 'ranked match' where the text saying waiting for players to join game text is flashing do not count toward this achievement!
And finally, you do not need to get all of the kills in one game, and also kills count even if you quit out or disconnect from the game, I got all of my kills through a few games in different sessions, and unlocked the achievement after getting 128 kills in my last match!
Good Luck!