Jack of Hearts, Volatile Hero

The whole engine hinges on the death trigger, so the game plan runs backwards from how a creature normally works: you spend the fight trying to lose him at maximum size, not keep him alive. On death he deals damage equal to his power to every creature, which turns a small evasive attacker into a delayed board wipe whose fuse you get to set yourself. The power-up is the fuse. Wheeling your hand into three fresh cards is the sweetener, but the two +1/+1 counters are the payload, because every counter that survives to his death raises the size of the sweep he detonates on the way out. It fires exactly once, so the design is deliberately front-loaded: commit early, dump whatever an aggressive draw has left in hand, and stack the counters onto a body that already flies and already has haste. The cost-reduction clause rewards the greediest sequencing, shaving his mana value off the activation on the turn he arrives, so a fast enough start can cast him and begin arming the sweeper in one turn. Read across its pieces, this is a red creature engineered to trade catastrophically and take the board with it, using its own accumulating counters as both a clock and a stored blast radius. The volatility the typing promises is honest: he is most dangerous in the exact combat that kills him.