How do I kill the fiend of mar in curst gone?

I'm at Curst Gone and I'm trying to kill the fiend of mar. I have read that people use Annah, hide in shadows and backstab to kill it but I can't get it to work.

I use Annah. Use hide in shadows, sneaks up behind the find and attack but it's just the normal attack doing only 2 in damage. But I have read of people who do 100 damage per hit using this tactic. Any suggestions?