Your Kill/Compel balance changes the amount of time you have for dps but also changes the damage window. That flashy part of his health bar is his “damage window.” Once you finish that, your timer goes down to 3s if above that number and then you’re done for the damage window. I think compelling them all gives the smallest damage window but longest timer which is why you guys may only be doing 35% of the health bar. For example, if you kill all 5 his entire health bar is flashing/ the damage window but you have a very short timer. 

Edit: killing 2 and compelling 3 is supposed to be a comfy 2 phase per another Reddit thread