To fight Gorea's second form, in the first fight with Gorea, destory his arms so that he reveals the Seal Sphere. Jump to the outside ring and Gorea will not attack you. Shoot the six colored symbols in the following order and with the corresponding weapons:
1. Yellow (Volt Driver) 2. Green (Battle Hammer) 3. Orange (Magmaul) 4. Blue (Shock Coil) 5. Purple (Judicator) 6. Red (Imperialist)
This is taken from the Alimbic Prophecies collected via scans throughout the game. After activating all six symbols, you will get a message from your ship. After defeating Gorea, you will be transported to the final battle arena for the true final fight.