Disclaimer: I wrote this 2 years ago and originally wanted it contributed to a much fuller guide. Since it still hasn’t seen the light of day, here it is:
This is tough @_@. Golden Mage Knight got nothing on Pluto and Eternity ;_;
Anyways, I like to play as Milla so I select her for the playable character. However, that shouldn’t change the strategy I will tell in a moment. If you play multiplayer, I wouldn’t suggest having more than 2 people on this fight so that you can take advantage of linked combat. Maybe you can take turns if you have a non-AI party of 2+.