Rain of Blades

One mana of white that taxes only the creatures committed to the attack, and that conditional is the entire balancing logic. Sweepers that scale with the board (Pyroclasm, Slagstorm, the asymmetric burn that hits everything) cost the caster real flexibility: they catch your own creatures too, and they fire whether or not anyone is attacking. This trades that breadth for a defensive condition baked into the spell itself. Cast in response to a swing, it punishes an overcommitted attack while leaving your untapped blockers and any creature that stayed home untouched, so the asymmetry is purchased by waiting for the opponent to walk into it rather than by paying more mana. The ceiling is obvious and low: one point of damage clears tokens and the weakest aggressive starts, nothing more, and against a single fat attacker it does nothing of consequence. What it offers is a cheap one-shot answer to the go-wide problem at a price that fits in a tempo-light hand, the kind of one-mana white instant that exists to neutralize a swarm rather than to deal with threats one at a time. It is removal that only works on the offense's terms, and that restriction is exactly why it can cost what it costs.