What was the Greek Battle plan to defeat the Persians at Thermopylae

To hold the pass to precipitate a naval battle in the adjacent strait at Artemesion.

They lost the sea battle, withdrew from the pass and refought the sea battle successfully at Salamis.

it was important to destroy the Persian fleet at it both threatened the Greek cities which therefore kept their armies in defence at home, and also protected the Persian sea supply line.

Withe the fleet destroyed half the Persian army was sent home because it could not be supported in such a poor country as Greece, and the following year the Greek armies, their cities no longer threatened from the sea, were able to concentrate their armies at Plataia and defeat the depleted Persian army.