The Cliff Demon can be stunned and locked down to give certain classes more time. I have been able to defeat it with various different class combos in various levels, as I do farm this instance with different levels for the drops.
M/P 50/50 - Discharge, Flame, Lightning, walk back, Flame/Fire. If you can't defeat it in this cycle, run to the closest porter which ports you and leave combat and try again. It won't heal.
R/x 22/x - a little more tricky, take a potion right away, and try to get to a porter to leave combat and to prep for your assault. Premeditation, Sneak Attack, Assassin's Rage, and then try to get up behind him and spam your instant attacks as much as you can while kiting it to the closest porter. If you think you are closer to killing it, then kite it to a healing portal instead.
W/R 44/32 (I dual Wield instead of 2H) - Shout, Frenzy, Blasting Cyclone, Spam your instant attacks and kite it again to a portal (teleport if you think you can't finish it, heal if you do)
K/M 36/30 - I keep my Holy Light Field and Lightning Armor up, if you are not prepared, use Shackles to lock it down and prep. Use your Shield of Valor and Truth to lessen its damage, and spam your attacks, use Resolution as needed and as always kite it to one of the portals as you see fit. Since a Knight takes a little longer to kill it, its best to get it to a porter one first then heal up and try to kill it on the second go round.
Of course my M/P has the easiest time in killing it, typically lightning/discharge and one or two flames will typically kill it for me. But with practice I do believe any class has a chance to kill it, use your potions when you get 50% hps as it can stun you with one of its attacks.
Below is what the room's portals do. The "O" is the doorway in which you came in.
X1-O-----X2
X3-------X4
X1 - heals
X2 - ports you to X1 (doesn't always leave combat)
X3 - ports you to X2
X4 - heals
Hope this helps anyone having problems defeating the Demon. It has a guarantee blue accessory drop, some are of course better than others.
I have had it spawn 3 times for me in one game, and sometimes I don't see it for a while, so its very random.