So just got a reaper to spawn in the airforce base… but he’s surrounded by 4 rivals in the area. If I started dropping them first will he despawn? Or is he here to stay until I kill him?
It will stay until you no longer have a drop of blood in your body 
Seriously the reaper will stay regardless of what you do.
You can also start a battle, get him to 50% health, stop playing and resume the next day. You will restart at 50% reaper health.
Perfect thanks just wanted to make sure. I knew rivals and other enemies carried their scars until ya kill em. Just wanted to make sure I didn’t need to stay above the region level 20 or he would sneak off.
I guess the same applies to the Reaper, but if you destroy the weapons of a Tank or Harvester and go away, when you come back they are back online, but the health will remain the same as it´s been previous said.
With question answered, i’ll lock this topic. 
//Mod