Hidden Conditions
To start this PQ, you must talk to Nappa. He will be hanging out in Toki Toki City once in a while after you complete the Return of the Saiyans Saga.
You'll be fighting with Yamcha one on one. He should be easy enough to defeat. Tien and Krillin will arrive next and you have to fend for yourself for a few seconds. Nappa will then arrive to back you up. After defeating the two of them, Piccolo will arrive, then Kid Gohan, and finally Goku. If his health goes at 50%, he'll unleash his Kaioken power and will have increased stats. Still, you'll have a good chance of attacking him from behind since Nappa will keep him busy. As long as Nappa's alive (or didn't suffer that much damage), Goku might be revived and use his triple Kaioken to fight you. Defeat him while in this form to complete the quest and get a higher rank.
Remember that it's not over if Nappa KO's. Goku will simply move to another area and fight Vegeta, in which the same rules apply. If you don't think you can do this with Nappa surviving, let him get defeated by Goku. But do remember to at least damage Goku before he kills Nappa, it'll make the next fight easier.