So, I actually just had this come into play. I ruled that the enemy was reduced to 1 hp. Their original HP max was like 160 and they had taken ~90 damage.
bump up their exhaustion to level 4 (thanks, sickening radiance) and now their hit point maximum is reduced.
That said, I count damage up, not health down, for bad guys. Anyway, felt good for everyone at the table.