Well, first off, please don't use Elixirs inside fights. The same Elixir could've earned you a full bar of AP, which should result in more resources than winning that single fight.

Now then, Advents are single player, so winning them as a new player can be tough. Do you know about debuffs? How weapon grids work?
Sanathlig has made a guide which explains a lot of things. Like rerolling - did you reroll for a 100% Eidolon?