Each match is limited to 90 seconds, as indicated by the timer between the two opponents life-bars. Each second in Injustice is equal to about 2 real seconds.
Have more health than your opponent when the timer reaches 0 to unlock this achievement. If either of you is knocked out before the timer reaches 0, the match ends and you'll have to try again.
The easiest way to do this is in a local multiplayer match with a second controller. Start the match, hit your opponent a few times and let the timer run out. If you don't have a second controller, this can also be done easily in a single battle on very easy difficulty, or against a friend online.