you need to get some random battles under your belt, so you can level up, and plan you route on the sphere grid, the one which goes past the hp 200+ sphere's as you level up so will your aeons, then use the tried and tested aeon bullet shield first you need to power up your people and aeons (overrides at max with random battles) approach cast haste on your summoner then when the dark aeon is about to attack summon your aeon unlesh the override then when the aeon attacks it just kills the aeon not you
TSA