You'll control a Titan's "weapon". You have 5 abilities : cleave, electric discharge, heal, evocation and "divine shield".
I suggest you start from the bottom of Avalanche and climb it killing the mobs on your way, so you get used to your abilities by the time you arrive to the last guy.
Killing the 200 undead should be easy, you can pack them and spam cleave, or if you have enough mana and won't have health problem you can use the electric discharge.
For the 3 named ones it would be useful to clean a maximum the area around and only concentrate on them. Be sure to heal yourself and use evocation before every boss and DONT use evocate during a boss, you get hit 5 times harder during the evocation.
-Bythius the Flesh-Shaper is wandering in the bottom right part of Avalanche, he has 2 elite dog helping him but it's the easiest one to kill.
-Urgreth of the Thousand Tombs is in the middle left, not really hard too. More health than the other one but he is alone (not counting little undead around him). He does an AE attack, just step back a few seconds (maybe a few evocation ticks can help if you are not attacked by small undeads)
-Hailscorn is standing just above Urgreth. The hardest to kill. He sometimes raise skeletons to help him, more often than you can do you electric discharge thing. Moreover, since he has a lot of health and hits quite hard, I suggest keeping your mana to heal yourself and only spam cleave until it's over.
The "divine shield"-like removes you from combat and resets any fight in progress, the mobs will stop attacking you. It's more like a self divine intervention actualy