Picture


Caption: The Greeks (who were) fighting very bravely defended themselves against the Persians.

Why ἤμυνον 
were defending themselves, action viewed as taking place - sometime in the past
Had it been ἤμυναn then a better translation would have been:
They warded the Persians off, with the stress on the simple fact: they did it, defended their country against the invader, once and for all. Full stop.
Unfortunately for them they didn't.
 
They fought not only bravely ἀνδρείως 
They fought but very bravely     ἀνδρειότατα 
They fought as bravely as possible  ὡς ἀνδρειότατα 
according to the pattern we've seen before in:
They fought as quickly as possible  ὡς τάχιστα