NPCs can vary in strength from superhuman to regular human to fit your needs (since you can control those yourself, I'll leave that up to you), but if you do manage to kill one they will also revive next day the same way PCs do. The moment a fight starts, Rovers will deploy to break it up so the window to do damage will be short. The Rovers are indestructible.

In Bastions case, their guns should probably be nerfed already due to the no weapons rule. Punching still works though.