Unlock all characters in the game, then when you are in Story 22, win the fight versus Trunks with the Ginyu Force (once with Guldo, once with Recoome, once with Burter, once with Jeice and once with both Ginyu forms). Hercule will now be unlocked.

The first time once you get Gohan back on Earth. Beat Buu as Super Saiyan Gohan. When it's over he will be a Super Saiyan 2.

In order to unlock Super Saiyan 3 Goku, just unlock every character and their form (What ever you hear you DO NOT need Evil King Piccolo to complete this) and Goku's Super Saiyan 3 form should be available on the last fight.

Play as Piccolo and defeat Nappa.

Unlock Evil Buu (Gohan absorbed), then play as SSJ Vegeta or Fat Buu and defeat Pure Evil Buu.

Have Goku in the first position of your team when you fight Android #16 and Android #18 and defeat them.

Play as Android #20 and defeat Android #17.

Have Krillin in the first position of your team when you fight Android #16 and Android #18 and defeat them.

Play as SSJ Vegeta and defeat Android #19.

Play as Android #19 and defeat Android #20.

Play as Goku and defeat them.

Play as Vegeta and defeat defeat Ginyu-Goku and Jeice.

Play as Cell's first form and defeat Android #17.

Play as Perfect Cell and defeat Cell Jr.

Play as Piccolo and defeat Cell's first form.

Unlock Majin Vegeta.

Unlock Evil Buu (Gotenks absorbed), then play as SSJ Gohan and defeat Evil Buu (Gohan absorbed).

Unlock Evil Buu, then play as Gotenks and defeat Evil Buu (Gotenks absorbed).

Play as Fat Buu and defeat Evil Buu.

Successfully complete the Future stages as SSJ Future Trunks.

Play as Young Trunks and defeat Fat Buu.

Play as Super Saiyan Future Trunks and defeat Frieza.

Win the fight in EX Story1 as Android 19 And 20.

Play as Captain Ginyu and defeat Goku.

Win the Goten vs. Trunks battle.

Play as Krillin and defeat Guldo.

Play as Goku and defeat Recoome.

Lose the Goten vs. Trunks battle.

Don't unlock Vegito until the third round of story mode. If you have, then start a new game, and play through it twice unlocking everyone except Vegito, Hercule, Android 17 and Android 18. Then on the third round unlock all of these characters. Then on the fourth round of story mode, when you play against Buu-Gohan, kill him with Hercule. Then against Kid Buu kill him with Vegeta normal. Then in Battle mode, with Android 18, restart the gameboy until Android 17 comes up in Battle Mode. Kill him and then finally, Gokule will be unlocked.

Have Cell 2 in the first position of your team when you fight Android #16 and Android #18 and defeat them.

Although SSJ4 was not featured into Dragon Ball Z, it is possible to get this form of Vegeta, Goku and Gogeta (Goku and Vegeta fused by Fusion Dance). Unlock Goku, Vegeta and Veggetto at their strongest forms (Goku SSJ3, Vegeta Magin Vegeta and Veggetto SSJ). Then, complete the entire game including Trunks' Extra Story with only those three characters at their strongest form in any order.