-( Story related, cannot be missed )-
After Clank opened the Orvus Chamber, he was taken hostage once again by Lawrence and Dr. Nefarious and then given to the Valkyries. Ratchet and Azimuth respond to his distress signal and end up having to face the Valkyrie Training Grounds. Eventually, you will have to face Cassiopeia at the pinnacle of the citadel. She will either lob gas grenades at you from the front or the side, hover in front of you and shoot at you, or flee (twice as health diminishes and summons Winged Valkyries to take care of you). If you don't find the stage of what she's doing your preference, you can jump over to another platform and (most times) she will change her attack strategy up.
Near the end of the fight, she will simply try to fill you full of bullets. Just strafe and shoot some more and she should fall.
*NOTE: The Winged Valkyries were, for me, falling quite quickly from the Mag Nets. One net seemed to dispose of them well enough for me to concentrate on facing Cassiopeia as fast as possible.