short controlled burst trophy. ..

I just managed this one on my own. It's actually not that hard once you figure it out. Obviously it's best done on recruit. I used a Pulse rifle with magazine accelerator and firebomb launcher. The Aliens only come from one side at a time so you can stay focused on what's in front of you. One or two might come from the right when you start on the left, but the NPCS can usually take care of them (I lay a claymore at the barricade gap just in case). When the first wave comes from the left I drop a couple of firebombs (hand-held) on the first couple of xenos. Then I very easily mop up the rest with the Pulse Rifle. It's not long before Keyes shouts that only a few seconds remain, and that's when the Aliens start coming from the right. I saved the Pulse Rifle's four firebombs for this bit and fired them whenever two or more Aliens were close together. Just gun the rest down as they come, until the trophy pops. Not hard at all.