Polymorph is said to be a great spell, and I can see why: it can turn a CR 18 monstrosity into a CR 0 Frog, or something along those lines. My question comes from one specific paragraph in polymorph's text:
The target assumes the hit points of its new form. When it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form.
Say a fighter, level 11, attacks a creature that has been polymorphed into a Frog (AC of 11, 1 HP). The fighter has 3 attacks thanks to his Extra Attack class feature, so when the fighter uses his action to attack, he attacks the frog three times.
1 hit is guaranteed to exceed the 1 HP of the frog.
Would that attack end the polymorph spell, and the remaining 2 attacks be against the original creature's AC and HP? Or, would the polymorph end at the end of the Attack action, meaning the fighter got all 3 attacks against the AC 11 Frog?