Erthenia

Well, to be honest, if we look at the mechanics, with only 100 HP, she would die from one or two shots/spells.

However, she has an incredibly high Willpower stat and doubled dodge chance. And for melee units, she is virtually invulnerable, and with 800 mana, she can obliterate a small army with magic.
-- Oh, please don't! I won't do it again! Let me just leave, and all your problems will go away with me, -- said Ignatius, retreating towards his horse. The demon is stupid, and this should definitely work. Sometimes, to defeat evil, you need to use not only strength but also cunning.