Iron Fist, Living Weapon

The design hinges on a piece of aikido logic: buffing your own creatures is the trigger, and the redirected damage is the counter. Most creatures that convert spells into reach want you to cast at them or at the opponent; here the condition is any spell targeting a creature you control, so the pump spell, the protective instant, the aura you were casting anyway becomes the fuel that arms the fist for the turn. The granted ability throws damage equal to his power at any other target, which keeps the aim wide: not just an attacker or a face, but a planeswalker or a battle if the moment calls for it. That single tap is the honest limit on the whole engine. However many spells you chain into a creature you control, the ability still demands a to fire, so absent an outside untap he gets one shot per turn: a single well-placed combat trick that both saves your board and clears one problem, or reaches over the top for the last points. The window is narrow twice over. The tap ability only exists the turn you feed it and lapses at end of turn, and the obeys the usual summoning-sickness rule like any other tap ability, so he must have been under your control since your most recent turn began before he can point the fist at anything. He wants a red spell-density shell that already holds cheap targeted instants, given a legendary Warrior whose power scales the payoff.