Here is a fool proof strat that I was told, works like a Charm:
Respec to a smiter monk, get the following skills:
Bane Signet
Banish
Balthazar's Aura (not spirit!)
Symbol of Wrath
Healing Breeze
(Fill the rest of the bar with useless skills, like heal other spells)
Start off by casting healing breeze on yourself. Run up next to your mirror and cast balthazar's aura on yourslef. While standing next to it, cast symbol of wrath and then bane signet to knock it down. Cast banish over and over until it dies.
This worked the first time for me, and if you have enough points in Smiting it should work very well for you. I killed the mirror in under (estimate) 20 seconds after failing with many other strategies.
I cranked up his smitting skill and used a staff of 11-21 plus a few extras on it and then put on the skill bar heal other type skills. The mirror uses only what you have on your skill bar. I only used 3 on the bar at the time I faced my mirror and it took less than 8 sec to kill him. I was healed in the mean time too.
I used Zealot's Fire, Retribution and Divine Boon.
Spamming Divine Boon heals you and triggers Zealot's Fire which mirror don't mimic. Retribution gives some damage back but I don't target attack so Retribution does nothing to me. Leave other skills empty or fill them with useless skills. Remember, no other healing skills but Divine Boon and/or Healing Breeze if mirror won't mimic Breeze.
I just used healing breeze. Only thing I loaded on my skill bar. I just hit the space bar to attack and hit healing breeze whenever it expired. Beat him the first time I tried.