there aren't really any monsters that can threaten a warlock with an immortal demon in front of him unless you do something stupid
probably the scariest would be one with erratic AI or teleportation so they lose aggro on the demon
Warlock has a pretty easy time, but if you're not paying attention while teleporting around in p8 TZ and you teleport around the wrong corner, Archer's can all simultaneously shoot at you with fire arrows and drop you down to like 300 life, while of course your demon is hardly scratched lol.