Windstorm

The job here is narrow and the design knows it: a scalable answer to a battlefield full of flyers, priced so the green deck can pay exactly what the board demands and no more. Green's relationship with the air is its oldest weakness, the color that builds the biggest ground creatures and watches them get chipped down by anything with wings. Reach patches part of that, Plummet and its cousins handle a single threat, but a swarm of small flyers (tokens, a go-wide air deck, a tribe built on evasion) demands a sweeper, and green almost never gets one. This is the exception, and the X is doing the careful work that keeps the answer from becoming a generic board wipe: the damage only lands on creatures with flying, so the green deck's ground forces walk through untouched. Everything green wants to protect stays on the field while the air clears, which is the only reason a color built around fat ground bodies can afford to run a sweeper at all. Cast it for two and clear a wall of 1/1 fliers; cast it for five against a single large evasive threat and you have overpaid for a kill spell. The instant speed matters more than the rate suggests, since it lets the green player hold up the answer and blow out an attack mid-combat rather than committing on their own turn. It is a deliberately conditional card, useless against a creatureless or grounded board, devastating against the one thing green is built to fear.