This Achievement is easy to get on the final campaign level, the Altar. To unlock the Altar, 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 rocket launcher for this Achievement. It looks like a green rifle and fires shots that explode on impact with enemies or walls, killing with splash damage.
In the Altar 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 rocket launcher 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 20-30 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 10 enemies, the Achievement will pop.