Megan, you can do it, just dont get frustrated and prepare your units if you must with the dunes.
Took any OP elza friend and follow this:
Turn 1
You: Cheer, Full Break, Omni Veil, ATTACK ATTACK ATTACK
Maxwell: Genesis, end turn. Genesis with onmi veil is a joke, so not worry here. If Maxwell cast any other thing but Genesis is because you drained her much HP and activated her HP thresholds.
Turn 2
You: Provoke, Curaja, Call of the Void, Enfeebling Slash (VERY IMPORTANT), ATTACK ATTACK
Maxwell: 2-3 Destiny, 3-4 normal attacks. End turn. Enfeebling is the ace here, with that you must disease max. If you do it, you have the upper hand to win the trial in the next 4 turns.
Turn 3
You: Cura/full break, Cheer/Curaja, Omni Veil, Madness Rush (Enfeebling again if Max is not diseased) ATTACK ATTACK
Maxwell: Probably meteor, or sacred song and normal attacks
See if you need to cure or refresh the buffs.
Turn 4
You: DISPEL, Full Break, Cheer, Call of the Void, Madness Rush, ATTACK ATTACK (or use items to heal like X potion or even elixirs)
Maxwell: probably meteor or resurrection and genesis and normal attacks
At this point you should trigger any of maxwell's HP thresholds and avoid Rune (that is the attack that game over your party)
Turn 5:
Go all out with full HP
Maxwell: Endless (if you can give her HP to 30%)
Nothing to add here, its you or she
Turn 6:
Go all out again to kill her
Just keep trying and you will make it girl.