on level 300 start out like GeorgieVaz said above. pick sub-zero and spam the Y,Y,Y, combo, and freeze. now if your anything like me, you've failed a-lot on this level. you lose a lot of health on goro press start and retry. now this is where i'm going to tell you to STOP!!!
no matter how bad you're losing do not retry in the middle of the match. sounds stupid i know.
after 5 losses you notice after every win, you gain a little health back. almost so little you can miss it.
after about 10 losses, you gain 1/2 your health back after a win, & goro doesn't turn invulnerable before every attack or at all anymore. also by this point they aren't beating you down to the point that you can't move.
after 15 losses, by now i was making it to shaokahn every try. goro is a push over kintaro just jump Y,Y,Y, freeze repeat. for mileena a found dropping the freeze all together & dashing forward Y,Y,Y worked well, the same for shaokahn. (jumping over hammers as needed)
so. i know i suck i writing guides, but i use them enough i thought i should try & give back a little. hope it helps