Pretty self-explanatory and is the lesser of two evils. This and it's sister achievements (World's Finest and All Star) are earned by completing the S.T.A.R. labs found within Single Player mode. For this achievement, you'll need to have played and have finished all 240 missions in the labs that ARE NOT from the DLC.
The earlier labs are easier, and progress as you move along from character to character and each character has 10 individual labs assigned to them that progressively get tougher towards the end. For this achievement however, you don't need to earn more than one star or win the fight in order to complete them. Although, it wouldn't hurt to attempt to earn the 3 stars for each mission, while you're playing through this mode.
Note: It helps a great deal to play through battle-mode or training, until you learn simple combo commands that you can use. Also, only use finishers when you are absolutely certain that it'll connect, otherwise opt to rather use more meter-burn attacks and block often. Good Luck!