Diminish

The trick is that it does not subtract; it overwrites. Where most blue tempo spells bounce a creature or tap it down, this one resets base power and toughness to a flat 1/1 and lets the layer system sort out the rest. That is the wrinkle worth understanding: base-stat setting lives in Layer 7b, which resolves before everything that modifies those numbers afterward. Anthems in Layer 7c stack on top, and +1/+1 counters in Layer 7d add their bonus to the new base. Diminish a 5/5 carrying two counters and you get a 3/3, not a 1/1. The same logic punishes you against a freshly cast pump spell: a creature pumped by Giant Growth applies that +3/+3 in Layer 7c, on top of your 1/1 base, so you have spent a card to leave a 4/4 attacking. The clean targets are the ones whose size is its printed stat line, where overwriting the base genuinely guts the body. At instant speed for one mana, the windows are all about the current combat step: shrink an attacker into a chump, knock a blocker below what your own creature deals, or set up a swing where a single point finishes the job. The effect ends with the turn and the creature stays, so there is no permanent answer here; the value is entirely tied to this turn's combat math. Against a vanilla early drop it accomplishes little, and on a wide board it touches exactly one creature. This is base-stat setting as a precision combat instrument, not removal in disguise.