The most difficult part of the trainer, should you choose to do it, is keeping all 20 civilians alive. 10 are present at the beginning and another 10 show up on level 11. The easiest way to keep them all alive is to move them to a location where they are easier to defend.
Beware that on level 20, a spawn point appears in the room where the first civilians are and one hostile comes out. If you moved all of the civilians to that room, make sure to deal with that hostile at the beginning of the level.