OK many costumes to pick from. my best bet would be her poker face outfit. You have to have evrything though. hair, makeup, clothes, etc.

If i were u id just wear black swimming costume, get a wig from Matalan and you can get ldy gaga hair bows in clares acsesorys and then get black high heels from primark

get a leotard, peroxide blonde wig and fishnet tights and you're ready to go!!

all of the above are great, and then maybe you can stick that lightning bolt on your face. and make sure you've got some mega false lashes! have fun! n_n