Massed Throwing Axemen with the Axe of Muspell improvement shouldn't do too badly against Phoenixes or Stymphalian Birds. Ballistae should also serve the purpose. Hammartrolls would also be a good choice if Forseti is used, unless playing Odin of course, but his Ballistae would be more durable so it sort of evens out.
Of course, Egyptians have their Priests and Pharoahs to fight flying Myth Units and they do it very well. Isis and Set also have their own Phoenixes if they go with Thoth plus they can field powerful Priests and Pharoahs with Nephthys. Ra would be advised to go with Osiris and get Mummies and the Son of Osiris along with the second Pharoah.
With Zeus, try Hera for the Medusae (although their special doesn't work on flying units they do double damage to Myth Units, they have 60% Hack Armour and when upgraded to Matriarchs they have over 400 hit points...402 maybe?). Backing up with Manticores & Centaurs would help.
With Hades, upgrade your Gastraphetes with Beastslayer and use Manticores as well.
Poseidon would have to rely on massed Toxotes and Centaurs if he has them. Can't think of many options for him. Best to go Artemis and get Shafts of Plague if attacked by Phoenixes or Stmphalian Birds I would say.The Atlanteans just need to upgrade a few ranged units to Heroes to deal with flying Myth Units. Of course, Gaia and Oranos can field their own Stymphalian Birds if they pick Theia.
Hope some of that can help you to beat your friend. Good luck!
[This message has been edited by Centaurus (edited 12-02-2004 @ 06:13 PM).]