Hercule
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.

Super Saiyan 3 Goku and Super Saiyan 2 T
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.

Nappa
Play as Piccolo and defeat Nappa.

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

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

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

Android #17
Play as Android #20 and defeat Android #17.

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

Android #19
Play as SSJ Vegeta and defeat Android #19.

Android #20
Play as Android #19 and defeat Android #20.

Burter and Jeice
Play as Goku and defeat them.

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

Cell 2
Play as Cell's first form and defeat Android #17.

Cell Jr.
Play as Perfect Cell and defeat Cell Jr.

Cell
Play as Piccolo and defeat Cell's first form.

Chibi (Little) Trunks
Unlock Majin Vegeta.

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

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

Evil Buu
Play as Fat Buu and defeat Evil Buu.

Evil King Piccolo
Successfully complete the Future stages as SSJ Future Trunks.

SSJ4 forms
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.

Fat Buu
Play as Young Trunks and defeat Fat Buu.

Frieza
Play as Super Saiyan Future Trunks and defeat Frieza.

Future Android 17 And 18
Win the fight in EX Story1 as Android 19 And 20.

Ginyu-Goku
Play as Captain Ginyu and defeat Goku.

Goten
Win the Goten vs. Trunks battle.

Guldo
Play as Krillin and defeat Guldo.

Recoome
Play as Goku and defeat Recoome.

Trunks
Lose the Goten vs. Trunks battle.