If you load up Act III: United, make your way through the mission until you have to destroy the dark fountains. I suggest grabbing the crossbow(hanging on the tree to the right of the movie screen) and the sawed off shotgun(in the trunk of a car across from where Serena is). Then run up to the four fountains and enemies will keep spawning in three. back up towards the ammo box and light and take them out until achievement pops
