I'm not sure if there's anyone with this ability, but I'm thinking of a Dark Knight with a sacrifice ability. He's a tank with high hp, some armor, and maybe void armor, but has low melee attack. Every round he'd sacrifice his HP to make his attack stronger. For example in round one he'd have 10HP, 2Atk, next round (assuming his armor took all the damage) he'd have 9HP, 3At, and so on.