Using fiery sword (14-20 custmised) and grim cesta (+10 energy)
Swordsmanship 10 (+1 from helm)
Blood Ritual - He sacrifices hp
Defile Flesh - He sacrifices hp
Blood of the Master - He sacrifices hp
Malaise - You lose energy, he loses hp
Chillblains - You take cold damage (not much, around 30), he becomes poisoned
Awaken the Blood - He sacrifices 50% more hp, but will gain some hp when skill ends (about 90)
Parasitic Bond - Target loses hp, and you regain hp when it ends (about 90hp)
The mirror self was dead in about 52 seconds (so close to the bonus!!!) and probably killed himself more than I did.
I had about 75% hp when he died.