This Achievement is easy to get on the final campaign level, the Altar. To unlock the Alter, simply unlock all five sub-rooms of each room by reaching both the required kill counts of the sub-rooms and the overall rooms themselves.
You'll need the harpoon gun for this Achievement. It looks like a black and orange rifle with a white spearhead sticking out of the muzzle. Harpoons stick into walls and can then be jumped on like platforms.
In the Alter level, enemies spawn in continuously as long as the final boss lives. Just avoid the final boss and don't fire any shots. This prevents the regular enemies from firing, so they usually won't be able to hurt you (I did get crushed against a wall once). Only the boss will be able to kill you, so avoid him by double jumping and moving around the room.
Avoid the enemies and wait for the harpoon gun to spawn at the top of the alter. Once you have it (and preferably no other weapons, to be on the safe side), keep avoiding enemies until 10-15 ghosts have spawned. The more, the better. Try to keep them all in one general location, perhaps a couple of steps down the alter. Then just fire a shot into the step with the most enemies nearby. If you hit at least 5 enemies, the Achievement will pop.