Deathclaws and molerats that won't die. They aren't even this difficult to kill in the Fallout games! I've had them respawn after killing them, or take out some of my highest ranked dwellers with some of the best weapons. Come on! Please get this fixed. I've spent a LOT of caps reviving dwellers that shouldn't have died!
Also, dwellers that just start losing health for no reason. They aren't near an enemy, but they kill over.