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.
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.
Once 20 or more ghosts have spawned, kill a few until the Exorcism item (which looks like angle wings) appears. Fire it in the center of the enemy crowd and you should snag ten kills, causing the Achievement to pop.