The description of the achievement is to kill 50 aliens in one game using the fire barrel or electric fence traps.
Me and my brother found a easy way, after you get past the first hive and open to the electric fences you will have to drill one more hive then give it a minute or 2 and the aliens should start spawning again. Keep running them around and jumping over the fence panel on the floor. After just a couple of minutes the achievement should pop.
I did this with a coop partner which made it much easier as he could revive me if and when I went down.