Can you destroy a powerful creature with True Polymorph, Turning, and Destroy Undead?

Question

As an Arcana Domain cleric, I cast true polymorph on a powerful enemy creature (say a Kraken), its statistics change to match a lower CR creature; I choose an undead (say a Skeleton).

If I then use Turn Undead on the new creature, will the Kraken be defeated in its entirety (per the Destroy Undead feature quoted below; quote edited based on level scaling)?

when an undead of CR [4] or lower fails its saving throw against your Turn Undead feature, the creature is instantly destroyed.

Assume the creature fails all saving throws.

Answer 1

Turn 'em and Burn 'em

This will work exactly as described in your question. True Polymorph is concentration, so that will use up your concentration during this event. Turn Undead does NOT require concentration, so you can just as easily Destroy the Undead (which, given your ability to cast True Polymorph), shouldn't be a problem in terms of CR.

True Polymorph includes the clause:

If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form.

You have invested in your 9th level spell and gotten lucky in the failing of saving throw for it by the Kraken. Normally, getting to 0 HP would return the Kraken to form, but Destroy Undead doesn't kill or bring a creature to 0HP, it destroys them and bypasses the 0HP issue.

XP

This encounter began with a Kraken, and ended with the defeat of the Kraken. Even though you True Polymorphed the Kraken into a lower CR creature, you still defeated the Kraken. The True Polymorph Skeleton->Destroy Undead was HOW you killed the Kraken. Because of this, you should get credit for your ingenuity and luck and get the XP for a Kraken.

But I could see a DM saying that you killed the Skeleton, not the Kraken. When you True Polymorphed the Kraken, it was no longer a Kraken and never became a Kraken again. And it died when it was a skeleton and still never reverted.

The DMG does state:

Each monster has an XP value based on its challenge rating.

While the end state was a different CR, I would not agree with this and base the XP off the original encounter and not what a player did to defeat the creature. Punishing a the player for the use of their highest spell slot, clever tactics, and a lot of luck doesn't seem right.

Answer 2

It would turn back into a Kraken.

This is what the spell says:

The target assumes the hit points of its new form, and 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.

RAW says that it would return to the number of hitpoints it had before it had transformed.

You don't revert to the normal form as a result of dropping to 0 hp, you revert as a result of dying. Therefore, the if statement isn't even applied.

Being destroyed is the same as dying.