The magnet gun is the easiest way to do this one. Get a berserker low on health, wait till you see him start to glow and charge at you(this is the only point that they will explode). The berserkers are the larger green or orange guys, they kinda look like something out of Doom. Once they're glowing and all pissed off, hit em with the magnet gun then shoot the other magnet into a crowd, he will explode on impact killing everything around him. I found this easiest to do on Infestation in Black Market. Get into a smaller area and wait for them to come at you en mass, I killed about 6 aliens with one berserker, so it's pretty easy.