You had several options for this achievement.
You get it eventually by playing either Campaing, Towers, Online/local Matches or agains AI.
The quick (and maybe the easiest one) method for this achievement is to play 1 vs. 1 with a dummy controller, so you can boost your flawless victory progress. something like the practice room but without the infinite life. and at the end you had the chance for Fatalities, and if you had some Easy fatality it will be more quick your XP progress.
you will also be able to win these achievements during the process:
(Mileena has an easy brutality)
And to get more exp you can modify your Kombat Kard for extra bonuses.