She cannot be fought until Plantera has been defeated. I would advise creating a long arena, making sure you have plenty of room for verticality, and an accessory that allows for faster ascent, such as the Frog Leg. I personally would refrain from using infinite flight mounts since the attacks are fired in large spreads and in rapid succession, which can leave the player vulnerable. As for weapons, pick whatever you think is strongest. My personal choices (as a melee user) were a mix between the Flairon and the Terra Blade.
And pray to god daytime doesn't roll around...