The 300 Spartans holding off the Persian Army, who doesnt know about this?
Simple and short:
Hold off the Persian Army until the Victory Timer runs out and the Greek Fleet arrives and kills off all the Persians:)
You might need too organize your Formations of Spartans e.g. two groups of Pikeman in seperated defensive Positions.
I know the Greek Fleet is not that much histrically accurate, but i think it doesnt need to be totally accurate.
This is a great map! I managed this on my first attempt, but it really was challenging - both reinforcements saved me.
I blocked the passage and slowly moved up to another choke point where I blocked it and narrowed all their troops down to a bottleneck.