Use Vegeta and equip Babidi's Mind Control. When you get down to your last health bar, let the CPU hit you. Make sure you hit the ground. After you hit the ground, you will see a short FMV sequence and Babidi will appear and turn you into Majin Vegeta. You now have access to two of the best moves in the game; Atomic Blast which takes up nearly an entire health bar, and Final Explosion which is the most powerful attack. If you manage to pull off Final Explosion, you will get a one hit K.O., even if the CPU is at full health. However, the charge-up meter for Final Explosion grows incredibly slow; and unless you fill the meter up completely, the only thing you will do is destroy Vegeta. To fill up the meter faster, do not rotate the Analog-sticks. Instead, repeatedly press X. This is the only way to fill the meter without destroying yourself.