*On Co-op, you can have one player kill the grunts while the other player hangs back to get that profile the achievement.
man am i glad to be a member of this site, lol i was going crazy over this "dont kill grunts" on 'two betrayals.' maybe on a lower difficulty, but on heroic it seemed like i was just getting punished while passing these grunts. doing it co-op will make this like 100% easier! bwahaha, i got to about 900/1000 fairly quick but this achievement was starting to get me angry
EDIT : i thought i posted this on the achievement guide sticky, whoops :P