Well, Phoenix and Apocalypse are hitting really hard. Got one win with elixirs and one without. Seems like Mordred with Trial by Jury for Apocalypse is good and Jeanne with desertion for Phoenix plus it's worth taking Balor (I kicked out Nike for her) for Phoenix even if dark isn't your main element.
My team for Apocalypse is Mordred, Shiva, Belphegor, Nike, Gabriel, Belobog and Uranus (last two because I am leveling them up)
For Phoenix almost the same, with Jeanne instead of Mordred and Balor instead of Nike.
I am going to aim for maxing both eidolons this event.
If that's your first SSR event eidolon, in my opinion, you should try to get it as high as possible as soon as possible so you can complete expert stage.