Choose Zone 1 on medium difficulty. This achievement pops up immediately when you get it. The person that presses is the coop weapon that will be chosen. Just wait for when there area lot of infantry coming.
The flamethrower should be used when the massive amount of guys are running away from the mammoths. Just have you and your partner on each side of the door as they run through your flame.
Other than that good areas are at the end of Area 1,2,4 and the boss at 5 since he spawns spiders and helicopters. Remember this pops up immediately when you get it.