I would just add full health at the start of every single level, that would make a dramatic improvement in fairness.
As far as enemies being grouped too hard, there is a couple spots. One in particular I thought was too hard - the one spot where there is two elevated corners in the room, and you through like not even joking 40-50 monsters at once. You got to go through a door, take a corner and deal with all that.. That could be nerfed a little bit, but NOT A LOT! But i did have to save scum, which is save after every kill without losing health until I picked them all off. That shouldnt be required. Im pretty good, I shouldnt have struggled THAT much. But yeah, going like 8 levels in a row with 20 health, and not being able to get hit not even one time was the majority of the pain I experienced. Fix that and your good. Just put health at spawns. On all 50 levels or whatever.