Keepsake Gorgon

Deathtouch on a five-toughness body makes a wall that trades up with anything and survives almost everything, and that patience is the point: the creature exists to hold the ground while a delayed removal spell sits stapled to it. Monstrosity converts surplus mana into a kill, but the seven-mana activation is the price the design charges for folding a destroy effect into a body that already blocks the world. Five to cast plus seven to monstrify is twelve mana before anything dies, and that activation is exactly that, mana you sink in rather than a second card from hand. The total rate is not the story; the sequencing is. The deathtouch body stabilizes the midgame once it lands, then the activation buys removal after you have hit your land drops and have nothing better to spend the top of your curve on, all without committing a separate card to the effect. And because monstrosity carries no timing restriction, you can leave the mana open and flip during an opponent's combat or end step, killing a creature at the moment it hurts most instead of telegraphing it on your own turn. What limits the payoff is that the flip is a one-time event: monstrosity sets once and stays set, so the destroy clause fires exactly once. You get one dead creature and a slightly bigger blocker, and then the payoff is spent and the wall goes back to doing what it does best, which is nothing in a hurry.