Trigger made an excellent thread on this.
Hope it helps ya!
http://mabination.com/mabinogi/guides/3661-besting-your-doppelganger-how-achieve-flawless-victory-g10-final.html
Possessed Cai=/=G10 Finale >:
Advanced Magic > Blaze?
Ice Spear works to keep him locked up, if you have it.
Maybe try weakening a bit, then load FH while a pet distracts him, so you can finish him off with FH.