Emperor Frieza!
Win: Reduce Frieza's health to a certain amount
Lose: Player HP depleted, Goku is defeated
Frieza's first form is somewhat a pushover especially if you're training hard prior to this mission. He has particularly nasty combos and follow-up attacks he can pull. Just keep attacking him until his HP is critically low. This will trigger a cutscene where he'll immediately transform to his final form.
Finally! Raging Super Saiyan!
Win: Reduce Frieza's health to a certain amount
Lose: Player HP depleted
After the events leading to Krillin and Piccolo's death, Goku has transformed to a super saiyan. You have to help him fight Frieza. Though your character will appear battered, he/she'll be in full health when fighting Frieza. You don't have to worry about Goku since he'll be more than enough to deal with Frieza; however, make sure to be aggressive as well to end the mission quickly.
Final Battle! My Full Power!
Win: Deplete Frieza's health
Lose: Player HP depleted
Now, the final battle is at hand. However, it may be anti-climactic since Goku's quite overpowered already at this point. Just attack Frieza and deal as much damage as you can to end the battle.
Cool the all lot