I just ascended on my FIRST try with my Monk primary. I wanted to let all of you know how I did this (pretty simple actually):
Equip only 2 skills:
1. Zealot's Fire
2. Divine Boon
Turn on Zealot's fire, run in close, and spam Divine Boon. Divine Boon is a 1/4 second cast time spell, so you can cast it really fast, depleting his health faster than he can attack you with normal weapons.
I decided to look at all my skills for ones that the mirror wouldn't know how to use properly (or use conventionally) but that could do a lot of damage. I decided Zealot's Fire was a good skill, but I needed another spell to combine with it that had the shortest casting time possible... Divine boon it is. I almost think using Divine Boon in this way might be considered an exploit, but one thing I've learned about this game is that you have to think unconventionally to win. One man's exploit is another man's winning strategy.
Hope this helps someone ascend.