Wild Beastmaster

The whole engine sits on a feedback loop that begins with nothing: a 1/1 that, on the declare-attackers step, hands every other creature you control +1/+1, which is a rounding error until you raise its power first. This is a multiplier wearing a creature's body, and the multiplier scales off its own power, so the payoff climbs quadratically once you start pumping it. Drop a single +3/+3 boost on it in response to the attack trigger and a wide board of small creatures all swing four points larger; stack two and the numbers run away faster than most decks can answer. Crucially, the buff lands on each other creature you control, not just the attackers, so mana dorks, untapped blockers, and summoning-sick creatures balloon too, which matters for a board that has just entered or been held back from the attack. That is the tension: it does almost nothing alone and ends games the instant it has help, a combo enabler in a Shaman costume. The constraint is the attack trigger itself. The pump only fires once it is declared as an attacker, giving the opponent a full window to kill a 1/1 before it ever turns sideways, and a 1/1 is the easiest thing in the world to kill. Stock the deck with cheap power-boosters and a board already on the table, and it is a one-card overrun; leave it exposed, and three mana has bought a creature that dies to a stiff breeze.