Perform every fighter's forward and backwards throws.
This was played in Practice Mode for an easy demonstration. In order to get the trophy or achievement you have to do this in any mode that involves real fights (I recommend ladder mode).
Actually, this is pretty simple. You just need to keep the overview. I suggest to perform their forward and backwards throws in the order I did. Play the ladder on beginner. When entering the next match you pause the game and select the next fighter. Do this for the first 8 matches, then go back to the main menu and start with the next 8 fighters. It helps to set the amount of rounds per match down to 1 in your general gameplay options (main menu).
In total there are 28 playable fighters. Two of them (Cyber Sub-Zero & Quan Chi) are unlocked in Story Mode. Head over to my 'Back In Time...' Guide, if you need help with the final bossfight of the Story.