Even easier:

For Motaro, back off and use a hop kick (straight up jump) whenever you see him start to move, jump or teleport. He'll walk forward/teleport into your hop kick, chain it into your character's longest dial-a-kombo. Repeat, even if he blocks; odds are he'll just walk forward into it again. Just keep an eye for fireballs.

Against Kahn,back off at the very start of the round. You'll be out of range of his charge, and able to duck his fireball. Hit him with an uppercut whenever feasable. When he's down, run up to him and duck; he WILL attempt to attack as soon as he gets up. With a bit of practice, you can time your uppercut to hit him out of it; rinse and repeat until dead.

Smoke and Scorpion can use their spear as he rises for a free uppercut, too.

MK3 bosses = cake. :3

Want another slice of cake??.....

With Human Smoke in UMK3,unlock him,when you reach Shao Kahn,knock him down and quickly run against him and tap LP a couple of times and spear him then uppercut and repeat until he's dead in both rounds.When he gets up he always attacks,so when you LP him when hes getting up you are interrupting the attack frames and it'll cause Kahn being pushed back and it's the moment when you spear him and uppercut.