You're in way over your head. The nightmares in this cave must be destroyed, by any means necessary.
Annihilation introduces a totally new way to play the game.
Push deeper into the cave to reach points of structural weakness, plant powerful bombs, then get back to the exit before they detonate. The Exterminator can stop horrors from spawning and is immune to gas, helping keep your escape routes open while you complete your mission.
This isn't going to be easy....