Crossbow Ambush

Green is Magic's primary anti-air color, but it pays for the privilege with timing: most of its reach comes stapled to a creature, which means the body has to already be on the battlefield when the flier commits to the attack. This instant detaches reach from the body and sells it as a reactive trick. Hold up a single green mana, let the flier swing into what looks like an open lane, and turn a clean attack into a dead creature. Granting reach to your whole board is what makes it more than a one-target answer: it covers a swarm of small ground bodies that individually could never touch the air, so a mob of weenies suddenly bristles with anti-air on the turn it matters. The narrowness funds the cheap rate. It does nothing on offense, it answers nothing that is not attacking through the air, and the grant expires at end of turn, evaporating the moment the window closes. That conditional, reactive shape is the whole point: green's flying defense is usually a question of having the right creature in play at the right moment, and this hands the color an instant-speed way to manufacture that moment instead of building toward it.