Load up a Team Deathmatch on Ascension with 3 controllers. It will be 1 vs 2, you against the 2 other controllers. Go to gamemode settings -> weapon selection and disable everything else, leave the Bearbomb checked. Start the match, have the 2 controllers sit in the same corner, go there, throw your Bearbomb next to them and go near, press
again to detonate it, killing yourself and the 2 controllers. Achievement unlocked.