Piccolo vs. Goku

First off, you'll need to be controlling Piccolo to even start this substory. You can find it on Kami's Lookout after you've finished the main story.

Before you head out, double-check that neither Gohan nor Goku are in your immediate party. They can't be with you for this one.

Once you're ready, Dende will ask Piccolo what he wants to do. Piccolo, being the warrior he is, says he has some unfinished business.

When you regain control of Piccolo, make your way towards Goku's house. He won't be there, but you can speak with Gohan.

Gohan will tell you that Goku is off training in the mountains near the house. Head that way and approach Goku to kick off the fight.

You should already know Goku's moveset pretty well by now. He'll use his standard Kamehameha, a Guided Kamehameha, and some melee attacks. Just keep dodging and maintaining distance, and you'll be fine.

Win the fight, and you'll be all done with this substory!