Kron The Undying is really great. (He's a Legendary, so of course he is.) I only have a level 1, but even then he's got 3 magic attack, 10 health, and the Heal ability.

Combine him with a level 4+ Khmer Princess (Untamed) and/or Wood Nymph (Rewards), both with Tank Heal, and Kron can withstand a pretty heavy beating each round. He does cost 10 mana, so not the best fit for all matches (or even allowed with multiple rule types), but I always give him a strong look.