Originally Posted by
kaustos
There's a pretty easy way to totally cheese this fight. Bring Midnight to ~97% and throw some dots on him, then use run speed potions or sprint away and begin kiting him until 95%. Attumen is not linked to midnight and if you are outside of aggro range (which is very small at 80) he will just stand there out of combat when he appears while you bring the horse to 25%.
At 25%, they will combine as normal except Attumen will not automatically aggro- you get as much time as you want to regain cooldowns, mana up and heal before you pull him again already in phase3.
So, you get only Midnight in ph2 and a fresh start from OoC at ph3. Easymode.