When you are fighting Freeza in his fourth and final form, let a weak character like Kuririn or Chao-Zu. My preferred choice will be Chao-Zu. Do not let Son Goku, Vegita, Gohan and Piccolo die. After defeating Freeza in his final scene, the usual credits, showing Goku flying will be seen until Vegita appears. Vegita will then transform to Super Saiyjin Vegita and so will Goku transform to Super Saiyajin Goku. Thus, the ultimate battle among the super saiyajin begins. Super Saiyajin Vegita is by far the toughest boss in the entire game, even stronger than the final form of Freeza. Once you defeat Super Saiyjin Vegita, you get a new ending that is different from the normal.