Help with Thoarla

I got a Thoarla from one of my RO cubes (with her AM3) and I wanted to know if anyone had a good build for her.

Congratulations. If she really does have AM3, then you just got the best firearm in the game. If you're not using it on anything, then you're doing it wrong. Thoarla has no internal booster or booster slot, meaning your LG selection is limited, keep that in mind when building.
Uh, my Thoarla that I don't use much anymore had Pacifar LG, Thoarla AM3, Victory AM, and a Viper Shield.

Is it locked onto Thoarla? If not I suggest maybe choosing a different core.. Say maybe Baltheon... Or maybe Aquila o.O That way you're not strapped to using her. I can't offer much else in advice though; I don't have that AM3.

Slap them arm itself onto one of the higher tier air bds (geo f, garula, sadie, shaden, gwyain, etc) and tune speed on it. Pair with either 38 fly or x2 shaden bs and propeller bits.

Thoarla BD itself is alright too, but you'll be strapped for slots and have limited LG selection. Still, it has some carts that may be of interest, and Jupitizer can be useful in maps with a high ceiling.

EDIT: Forgot they work great on Baltheon too.