I did it on co-op yesterday, but we restarted every time one of us accidentally killed a grunt (it took a very long time), so I can't say, but I know that for Popcorn.gif you need to kill 100 each, so maybe it's the same principle but instead without killing
Oh and a tip for "The Little Guys", turning on Grunt Funeral and Boom helped a lot when something else kills a grunt because it wipes out the whole lot of them for you. Such as at the start of the level, just hide and wait for a grunt to get killed by a sentinel and they should all die. Just be careful with grenades and the like
[Edited on 11.18.2011 10:50 AM PST]