Mordenkainen's Polymorph

The instant-speed answer to the polymorph question, and the timing is the whole point. Older transformers asked you to commit at sorcery speed and telegraph the play; this one holds priority, so it can intervene mid-combat, on the stack, in response. Pointed at an opponent's enormous threat, it overwrites the base power and toughness down to a flat 4/4: not removal, since the creature keeps living, but a way to defang a would-be blowout attacker and demote it to a body your board can trade with. Pointed at your own token or small creature, it is a combat trick that grants a 4/4 frame plus flying evasion. What it does not do is the clause worth internalizing: setting base P/T is not the same as erasing modifications. A +1/+1 counter or a Giant Growth applied on top still counts, layered over the new base, so a heavily buffed attacker does not simply vanish to 4/4. It leaves the target's abilities intact as well; this is not Turn to Frog. The one thing it strips cleanly is creature types, replacing them all with Dragon, which quietly severs tribal synergies and blanks anything that keys off the original type. The change expires with the turn, so its whole value is the window it opens and the reaction it forces: a tempo lever, not a permanent line drawn through the board.