Grim Contest

Most fight spells in green and black pit power against power: each creature deals damage equal to its strength, and a glass-cannon attacker can punch through a body far larger than its own survivability. This one flips the axis. By keying both halves of the exchange to toughness, it rewards the wide, defensive body over the aggressive one, and quietly rewrites which creatures are good at fighting. A fragile 5/1 that wins every power-based fight loses here outright, while a 0/4 wall that does nothing in combat trades up into something twice its size. Black and green both field stout, low-power bodies as a matter of course, so the spell tends to find a willing combatant already on the table. The instant-speed clause is doing real work too: held through an attack step, a defending toughness-heavy creature can ambush an incoming threat without ever leaving the ground. The catch is symmetry of risk. Both creatures deal and receive damage, so your own blocker can die alongside its target when the math runs the wrong way, and an opponent holding a shrink effect can lower your chosen body in response to turn a clean kill into a wasted card. The design idea is a single substitution: toughness for power, reading the board through the stat that fight spells usually ignore.