I don't remember it being so hard. I remember I used to cast Wall on one of my heroes, then I would just cast some massive damage spell on him and reflect it to Asura.
The amout of damage I would inflict was greater the what Asura could heal... Maybe I'm forgetting something, since it's been a long time since I last played it... Or maybe I was just leveled up already when I fought her...
I've seen the future, brother. It is murder.