[@Mega Birb] No. If Abigail attacked Artoria with a dream shot, Artoria could punch the dream fire into a dreamfire animal girl, and steal the power to attack with dream powered fire.

Basically, I looked at Emu's weapon stealing and went "I want in on that".

"*this ability also causes attacks to be treated as type appropriate before considering whether they would do damage. Ghosts, pure energy beings, and immaterial concepts can all be punched into cute animal girl-ness by Artoria's reality warping obsession."

Now, a shapeshifter would be able to avoid Artoria, if it was able to shift out of animal girl form before Artoria steals their powers, but seeing Artoria's power stealing functions upon sight, the best defense against Artoria's power stealing is "don't let Artoria punch you hard enough that it would kill you if she wasn't transforming the damage done into transformation magic isntead".