I remember that guy! One of the hardest fights in a no losses challenge. The thing is you can't stop him from casting geyser on you, so don't try. Just deal as much damage as possible as fast as possible, and then pin his last stack in the corner with summoned thorns and let him use up all his mana. Then you begin the arduous process of resurrecting your guys.
There's really nothing strategic about fights like that, it's just a grind. Unless you're adamant about 0 losses (which I can understand

) then it sounds like you did well.