Spell Combo: Various Conjurations and Polymorph

[No brains]

If a friend casts, let's say Conjure Elemental, and I cast polymorph on the summoned elemental, what happens when the polymorphed elemental reaches 0 hp? Does it revert back to elemental form? Does the spell end altogether? Are there any other consequences I'm not thinking of?

[Sinon]

I don't know of any reason the Polymorph spell wouldn't function normally -

The elemental is a legitimate target of the spell.

I could see a DM ruling that, in spite of being summoned, the create is not "willing" and allow it a save.

I wouldn't; the investment of two spell slots and the opportunity cost of two characters' concentration seems fair.

[No brains]

I don't know of any reason the Polymorph spell wouldn't function normally -

The elemental is a legitimate target of the spell.

I could see a DM ruling that, in spite of being summoned, the create is not "willing" and allow it a save.

I wouldn't; the investment of two spell slots and the opportunity cost of two characters' concentration seems fair.

My confusion comes from the fact that when the polymorphed elemental drops to 0 hp, it's unclear if that ends just polymorph or also the conjuration.

If the elemental persists after its animal form is reduced to 0 hp, it's a pretty good combo. All an earth elemental would do is bash stuff and take hits anyway, so allowing it to take more hits and bash for longer with little opportunity cost is cool.

But if the animal form reaching 0 hp also counts as the elemental reaching 0 hp, then the combo is pretty worthless. A triceratops is largely inferior to an earth elemental, so if a caster can only have one before it hits 0 hp, it's better not to waste polymorph. Maybe the strat could be salvaged by the polymorpher stopping concentration before tricelementops drops to 0.

[Blood of Gaea]

You are correct, the polymorphed creature would turn back into an elemental, it wouldn't be dispelled. The only consequence is that there's a decent chance whatever you polymorphed it into isn't as useful as the elemental was. Considering your investing two spell slots, I also don't see why a DM would have an issue with it.