Hire mercenaries to raise an army for the short term, preferably HAs to harras the rear of Alexander's army. Spread rumours to Greece and the rest of the empire that Alexander is dead and the army defeated. Your first objective must be to survive, even if that means sacrificing the Hellenic Empire. Remember that your enemy's enemy is your friend. Don't be afraid of making deals with surrounding states, promising land for armies and alliances. Then advance to attack Alexander in India. Make a pact with the Indian princes. Like every phalanx, if attacked from behind it will collapse.