Nappa: Beat him with Vegeta in Dragon World
Raditz: Beat him with Goku in Dragon World
Recoome: Beat him with Goku in Dragon World
Captain Ginyu: Beat him with Vegeta in Dragon World
Frieza: Beat him with Goku in Dragon World
Android 20:Beat him with Goku in Dragon World
Android 18: Beat her with Krillin in Dragon World
Android 17: Beat him with Piccolo in Dragon Waorld
Android 16: Beat him with Goku in Dragon World
Cell: Beat him with Goku in Dragon World
Great Saiyaman: Beat Cell with Gohan in Dragon World
Hercule: Beat Majin Buu (fat) with Great Saiyaman
Videl: Beat Super Buu (with Gohan absorbed) with Hercule
Future Trunks: Beat Vegeta on Namek with Kid Trunks
All the Buus: Get all of the dragonballs in dragon world and wish for Babidi's spaceship. Then do any of the stages in Babidi's ship to gain kili. If you have enough kili you can unlock the Buus automatically (a capsule would show up when the kili is being collected at the end of each stage).