Dragon Ball Z Budokai HD Collection

Yamcha has a pretty short Dragon Universe at a mere three fights, so this one won't take you long at all. Since he's default and all the key points are marked on your map, there's really not much to this, but a quick breakdown anyway.

Saiyan Saga:
1. Visit Kami's lookout. This gets you some no-effort experience.
2. Plains, center of the main continent. This gets you a Spirit Ball capsule, and then kicks you right out to the map if you want to equip that.
3. Same spot, versus Saibamen. This is a "You can't defend!" fight, so no blocking.

If you've not watched DBZ, it needs to be noted here that Yamcha gets killed by a Saibaman in the show when it grabs onto him and self-destructs. This carries over to Budokai, too-while it does standard damage to every other character in the game, getting hit by a Self-Destruct while you're playing as Yamcha will one-shot you. Just get up in the thing's face and interrupt its start animation for that with normal attacks-alternately, if the timer goes down when it starts the move, and it can't grab you in time, you'll win immediately.

For completionists: If you're after all the capsules (no achievement for this, though), you can get Puar's Wish (which reduces how quickly Yamcha burns through ki in Hyper Mode) after this fight on your second runthrough.

Cell Saga:
4. South City. This is a standard fight against Dr. Gero.

The good doctor doesn't put up too much of a fight, so just deal with him like one else. This'll unlock him. Additionally, if you have a Senzu Bean, use it during the fight to unlock one of the Story Reenactments in Dragon Universe's bonus menu. (No achievement tied to these either, and they're just tiny sound bites from the show, so not worth going out of your way for, most likely.)

Buu Saga:
5. Center of the main continent. Yamcha just reflects on the World Tournament happening.
6. World Tournament stage, south side of the map. There's one of two fights here:

-Normally, this is against Tien. It's a normal World Tournament fight, which means ring-outs are in play.

-However, if you poke around the map during the earlier sagas, there's a conversation during the Saiyan Saga (mountain range on the southwest part of the map) where Bulma kicks Yamcha out, and several all over the place during the Cell Saga about Bulma and Vegeta having a kid. If you see these, Yamcha instead challenges Vegeta to a fight at the tournament. Vegeta's a lot tougher then ol' three-eyes, and if you're a lower level you're not going to hurt him much at all, so just go for a ring-out. If you're speeding through this mode, you probably won't see this anyway, but it's recommended you skip it for an easier time.

If you're after the Dragon Balls, the radar is directly north of your starting point in the Saiyan Saga. (And again in the Cell Saga, but it'll carry over).