Defeat the Marzen 64.
Grab the flying clanballs released by the mech and throw them back at it. Eventually the mech will charge and shoot a laser beam, which can also be caught and thrown for damage. Deal enough damage and the mech will be defeated.
After releasing five of the orange clanballs the Marzen 64 will charge up its laser and fire it at Marina. The gem appears when Marina successfully catches the beam of the laser, however it will be impossible to grab if you're not positioned correctly. To obtain the gem, position yourself just slightly to the left of the center between the mech and the right side of the screen. When Marina grabs the laser the gem will appear and should be collected automatically.