[Jasonft]

If you want constant vigilance to be the theme, have the numbers build as the summoning event night goes on. Each hour there is another 'pulse' and a certain number of monsters (percentage or multiple) of the existing monsters in the area spawn.

If you manage to find and kill the Boss for a certain area the summonings stop completely for that area. Bosses are intelligent and know they need to hide to build up their forces however - finding them early in the night isn't easy.

Then you have the *really* sneaky bosses who leave things behind to give them an advantage the next wave event. Since you say the monsters want the humans to live you have a Boss monologue about it or have some humans 'accidentally' see a Boss doing this. Once the humans catch on they are going to expend great deal of effort to find these 'leftovers' and they will know every last hiding spot.