Complete all Tower missions.
-#300-
This Challenge is getting easier each time you lose! Instead of start fighting right away, you should get killed for at least 10 times. Don't choose retry in the Pause Menu manually. Wait until the fight is over and choose the retry option there. Your opponents will fight less agressive and will tend to taunt a lot more. Their AI difficulty will be reduced with each defeat. Moreover, Shao Kahn won't spell so many negative buffs on you. The buffs of your oponents will decrease drastically. Probably, the best thing is the health you can regain after each round. You have nearly no chance without a health recharge.
When it comes to the fighters, you should definitely choose Scorpion. His spear attack is very helpful and hard to handle for the AI. In bad situations you can also make use of his teleport to get away from your enemy for a moment. It seems that the AI is defenseless against cross-over jumps. Right after jumping from one side to the other you hit your enemy with one or two simple hits, followed by the special attack 'Spear' and banded together with the 'Doom Blade' combo.