Any kombatant will work as long as you can avoid the computer's combos, preceded by him or her running towards you. Your offense should begin with a flip punch and any combo that starts with using high punch. Keep repeating this on your first opponent and the battle will be easy.
Second and Further Battles
At the second battle and onward, the best kombatant to destroy the computer is Smoke. To catch the computer off guard, try executing a back flip and then a mid-air Smokeycut. After a successful Smokeycut, deliver a Spear and a good button-link combo. Also of note, a flip kick followed up with his Smokeycut succeeded by a Spear and a good combo is lethal.
To completely thwart the computer's plans, do a back flip into a Smokeycut, Spear, any good combo, and follow the combo up with a sweep to force the computer to do a back flip. Afterwards, use the Spear, any combo and a sweep. Repeat the Spear, combo, and sweep. Smoke wins!
Smoke and Sektor both possess the Teleport Uppercut, so flip at them so that they use it, and you can simply uppercut them, or use the aforementioned Smoke combo. If you're Kabal, however, you can use his Raging Flash followed by an uppercut, or jump punch and mid-air Gas Blast.
Cyrax will use his Exploding Teleport if you attempt to throw a projectile. Afterwards, he'll throw his Green Net. To catch Cyrax, all you have to do is throw your projectile, and block his Green Net. Throw your projectile again, preferably a Spear if you're Smoke. You can also use Smoke's Smokeycut and Spear combo after you've blocked the Green Net.
Avoid getting too close to Motaro, and never throw projectiles at him since they reflect at you! The best kombatant to use against him is Kabal since Kabal's high-punch combos with his hookswords will stick to Motaro. All you have to do is begin with a flip punch and quickly execute High Punch, High Punch, Down + High Punch.
Beware Shao Kahn's Sledgehammer! Start the match crouching until Shao Kahn does his Upwards Shadow Charge. Once he's airborne, quickly uppercut him, and run towards him and execute your best combo and a projectile. You can crouch again, and wait for his Upwards Shadow Charge so you can perform the counter again.
If you're Smoke, you can catch Kahn with the Smokeycut and another Smokeycut, Spear, and a combo of your choice. Take advantage of the double Smokeycut since Kahn is the only character you can use it on.
With Kabal, you can wait for Kahn's Upwards Shadow Charge, uppercut him, run at him, press Low Kick, Low Kick, High Punch, High Punch, Down + High Punch to send him airborne, do a jump punch with a mid-air Gas Blast, and do another Gas Blast when you land. Return to the crouching position and repeat. If you really want to be technical, you can do the Raging Flash before uppercutting Kahn and the rest of the combo after Kahn's Upwards Shadow Charge. Shao Kahn will be defeated.