When I talk to Crystal before the Shiar mission, she asks you to find out where did Medusa go when she tried to retrieve the ultimate nullifier. I thought that would be a secondary objective, as with similar tasks on previous acts. However I don't get any additional objective after that, so apparently it is the same thing if I find out where Medusa went to.
Also I noticed there is the following glitch to keep gaining exp. When you finish :blackwidow: treason objective, you can go outside valhalla and inside again, then talk to weasel, after that walk towards the exit. As you go away, the explanatory dialogue will show up once again giving free XP.