Ok this might be the most difficult game i've ever played...but I seriously have had to have killed 100 people as many times as I have continued...this makes me think its in one life...thats just ridiculous
Actually, this is a fairly easy achievement. On the first level just wait in an area where a few of these guys come out of door ways. You may run out of time and lose a life but on your second life you should reach 100 "kills" easy. Auto-kick (LT) is your friend.