Dark elves rare Paragon is unkillable when his 2nd skill is boosted to highest level. Text reads an ally gets the buff but if he is the last man standing he can target self. 

Skill gives unkillable for 2 turns but when upgraded the cooldown also reduces to 2 turns creating a never ending loop of unkillable.

Not fun for the arena let me tell you. Especially those individuals who choose just him on defence. You have no option but to quit.