There is a very easy (and suicidal) way to get this. First of all, you must be playing on a CC1 or CC2 map (Caves or any map below it in the list in the lobby). Once you have a group of 5, load up a Co-Op Elimination and pull your grenades out. Just run up close and personal into a group of enemies - preferably around a corner so they can't shoot you before you get close enough) - and start tapping the trigger for quick grenade tosses. You should be killed or incapped pretty quickly and the grenade will go off in their faces. Achievement unlocked.