Another trophy that shouldn’t be terribly challenging to get but I have had a few people tell me it took some work.
First up you want to equip a grenade mod that has the “sticky” property somewhere on it. It does not matter what the other properties are (the mod I used were sticky rubberized grenades which seems wildly counterintuitive). Once you have this mod be sure to equip it and head to a low level enemy area. I personally recommend the Skaggs to the left of the starting base area of the game as they stay at Level 1 and 2 through your time on Pandora during the opening of the game. Once you get a group of Skaggs to spawn, give them a chance to bunch together (usually while trying to digest your characters face), toss a grenade at one of them and if the explosion catches two or more, you’ve got your trophy.
For clarification, I had a group of 5 Skaggs and when I stuck my grenade to one of them the explosion wound up killing 4 of the 5 on the first try so hopefully this will work as well for you and I did not just get insanely lucky.