The above comments are very discouraging ... Let me encourage you then.
I suck at fighting games, but finished expert mode without continuing on my 3rd try (first two were the 1st time I ever play the game, and was late, was a bit sleepy, and didn't pay attention. I got the trophy for it.
Anyways, here are some hints:
A- In general:
1) Set the game two three rounds (two will be enough but just in case). That's better just in case the CPU gets lucky and combos you.
2) As one of the above said, the CPU reads inputs, so press the input you want AT THE VERY LAST SPLIT-SECOND if you can. Preferably after the opponent does something.
3) Block often but remain aggressive. As soon as an opponent is in range, attack.
B- Specific characters:
These are the only characters that had potential at beating me:
1) Kabal and Reptile using their dash moves. Again, make sure to block (and then punish missed dashes with a combo or X-Ray)
2) Shang Tsung - Make sure to attack from above quickly, and then land a combo.
3) Shao Khan (very easy TBH) - He has three types of moves, all of which can be easily avoided. His dash needs you to be close or jumping close, so make sure you're not. His spear has a fast recovery so just jump over it (do not attempt to attack him after this unless you're 100% sure you'll land a combo and not get hit). His hammer is very annoying, but can be easily avoided. If he tries to overhead smash you, just jump behind him, and if he tries to throw it at you jump over it and combo him (the throw has a delay so don't be fooled. Only jump AFTER you seem him throw the hammer.)
I did it using scorpion.
That's basically all you need to know.
Hope it helps, and all the best.
Peace.