Limited number of activations seems fair.
Another idea that popped to my mind (kind of an extension of other ideas here): make him immune to strengthen/weaken effects, and instead of heal 2 and add 2 armor, make him reset both power and armor after duel.
He will still do lot of damage when ignored, but at least won't get completely out of control if you can't answer him immediately, and you can still buff him for next duel (but only that, then he resets). As a compensation the annoying all-in decks won't get their Renew bricked, because he can't get banished by Mandrake.
I'm not sure whether I like this idea myself, it may be too much text / complexity on a single card, and may still have issues, but I thought I'll throw it in as a possible alternative.