Saibamen: In the Dragon Arena: use a Level 10-20 character, and battle opponents *AT LEAST DOUBLE* your own level. Eventually, Nappa will "Suddenly Break In". Defeat him, and the Saibamen will become unlocked. *Example: A level 25 Cell should battle a Level 50 anyoune. Once you "Power-Up" to Level 26, battle a Level 52,...and so on.
If you follow the above agenda, Piccolo should break in around the time you become a Level 38/39.
Death is but a door, Time is just a window,...I'll be Back!!!!!