Spawn him by trading a Mocking Beak (dropped off the Colibri in the surrounding area) to the ??? at G-8. Reccomend you have a strong tank cappable of keeping and getting hate as Avalerion has a move to reset hate. Whm/rdm 90+ is also handy because Avalerion can and does hit hard, so its nice to have a healer who doesn't easily run out of mp. Best to fight as a quartet, but a trio is possible.
Hope this was helpful and feel free to correct me if I'm wrong.^^