To obtain this achievement you need to play and win a WrestleMania match between the Undertaker & Sting in WWE Universe Mode. Now when you begin the game Undertaker is already unlocked, but for those who did not purchase the Accelerator DLC, which is included in the Season Pass and sold separately, will need to unlock Sting by winning 84 matches against the A.I.
You can view your progress towards unlocking Sting by going into "Options" from the main menu and then "Unlockables" and navigating through the listed superstars to Sting which will display your number of wins out of 84.
I have verified that despite the wording in the game that wins in both the "Play" Mode and WWE Universe count towards unlocking Sting. Thus it is recommended that you get wins in WWE Universe en route to obtaining:
If you have completed all other achievements for playing in Universe or as other superstars you can always go into the "Play" menu and setup an easy to win match (finisher matches are popular) and continue to hit "Rematch" at the end of every match and it will continue to count up wins for you without you having to setup a new match with long loading times after each and every win.
The type of match you win with either of the two superstars doesn't matter as long as it is against the A.I, at the Wrestlemania PPV (March event) in WWE Universe Mode.