When you see large groups of grunts and jackalls you usually don't need to use grenades because they're so damn weak but in legendary every cheat helps. Throw a plasma grenade and stick it to one of the low ranking grunts(yellow or red bio suits not so much on the silver ones) The grunt screams and runs to the big group of guys screaming "GET IT OFF! GET IT OFF!" then he blows up and kills the rest with him.