Spawn tracking = more control...schedules?

Im having issues of spawning enemies and keeping track of them, this mainly
arises from the way i need to spawn in bosses or end lvl cutscenes after
so many enemy waves. Im not sure if i should set a schedule(which im not familiar with)
or put a counter system on them so either when they die or go out of camera view.

Then run a function based of the status of the enemy life count to spawn in a boss, ect

just wondering if someone could point me in the right direction, im prototyping from
behaviors right now, but now since we have it the way we want it, im re writing the code
so i have more control over it. So maybe when i restructure it i might find a place
for keeping track of enemy death.

thx