Plummet

Green has never been allowed to kill a creature on the ground, but it has always been allowed to swat anything in the air, and this is the cleanest expression of that rule. The color's removal is gated by a permission structure as old as the game: you may answer fliers, fatties (via fight), and artifacts and enchantments, but a vanilla 2/2 standing in your way is somebody else's problem. By restricting the target to creatures with flying, the design hands green an unconditional kill spell at instant speed without violating that boundary, since the thing being destroyed is, by definition, the thing green is permitted to destroy. The two-mana cost and lack of any rider make it a deliberately plain answer; there is no card draw, no life gain, no reach attached, because the whole point is to give green a no-frills response to the one threat type it is supposed to handle well. It sits in a long line of conditional green removal that trades flexibility for efficiency, from Hurricane's mass air-clear to the various reach-granting blockers that solve the same problem on the battlefield rather than the stack. The instant timing is what earns its keep: held up across a turn, it punishes an opponent who commits an evasive finisher and turns a dead-on-board green deck into one that can answer the exact threat its creatures cannot block.