OFF
Christina Aguilera (Can't Hold Us Down or Dirrty would've slayed. This is vulnerability, not fierceness)
Taylor Swift (I don't get fierce vibes from the song. But yet I think it'd was a little hard for you, right?

Sorry)
SAVE
Lady Gaga (great choice. This song is really fierce!)
Girls Aloud (Hands down!)
Close Call: Katy Perry!
I am basing my votes on how 'fierce' the collaborations are.