400+ damage per kamikaze cultist is insane, and unless you build for 100% dodge, 2 cultists hiding in hordes of enemies and flashy effects could easily one-shot you in a long endless run. It's way too punishing.
Maybe consider something like... damage cap for multiple explosion from cultists. For example you could only take no more than 400 damage even if 3-5 cultists explode on you in 1 second.
Or make the cultist explosion applying a debuff instead of insanely high damage. Maybe a curse effect on player to increase damage player taken in 30 seconds, or a stagger effect that make player power -50% cooldown for 30 seconds.