I beat my mirror-self with my level 19 Ranger/Monk like this:
Put on all of my nature ritual skills that I had, and filled the rest of the spots with useless heal other skills. While the mirror is busy casting all of the nature rituals you are free to attack him all you want. Very easy!
I believe ranger monks have it the easiest. I loaded up with crap rituals and my last two I loaded Power Shot and Tiger's Fury. He just went right on down the line casting those rituals and I jsut alternated Power Shot and Tiger's. He was dead in like 5 seconds.
Don't even bother with Tiger's Fury or powershot. I did what justen did. I loaded up with nature rituals when I realised he used all the same stuff as I did, and as soon as the round started my mirror started casting each and every one of those nature rituals in order while I just shot him normally with my bow. By the time he was done and took a shot at me he had about 30hp left.