This achievement is given to players who, win or lose, complete 50 matches with 50 different superstars combined across the "Play" (Exhibition game mode) & WWE Universe Mode against A.I. opponents. The game will only count included superstars in the game (starting & unlockable) but does not count Created Wrestlers or Female Competitors as part of the 50 required to unlock this achievement.
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Edit: 2016-11-23
Additional notes provided below by nuttywray suggest that female superstars may indeed count toward this achievement. While I can confirm that I took perfect notes while working toward this there is the possibility that I had, having done it weeks before nutty, completed it before a patch corrected this issue. Given the two different experiences more information is definitely requested from our WWE brethren to see if it is hit and miss or if female superstars counting toward the achievement has been corrected!
Lastly, as nutty noted as well, if you're looking for quick matches the finisher stipulation is helpful, but also remember that you do not need to win these matches (although why wouldn't you!).