Phi-Bi

If you overlevel a ton and get lucky...

I don't wanna be a downer, but I honestly don't see any "proper" way to do this, at least presuming there isn't gonna be some change in what items or options for breeding are available.

Flying STAB should do well against most of her team, but Cradily will definitely be a wall. Ferroseed might be trouble too, but Cradily has Smack Down to hit your weakness.

Dartrix lacks any options for boosting via levelup and the Pokémon neccessary for breeding Curse or Ominous Wind on it aren't available at that point. X-Items aren't available to be bought either, but depending on which ones can be found lying around you might be able to get some mileage out of those. If push comes to shove you might be able to extreme PP-stall Cradily with a shitton of Super Potions...

That is all asuming you would go into the battle with just one Dartrix. You could always just breed a female one with some other bird and then charge Florinia with your bird army. They do all evolve into Ghosts! Although Cradily might still be an issue even then...

You could also hope for Oricorio to be available before Florinia.