275: What i did, was jump backwards and do Ermac's teleport while in the air, and repeat the same process over and over again. If you find the right timing, you'll be able to beat those Goros with no sweat.
269: You have to practice Stryker's gun for this one, because what Grune said really works. Spam the gun while watching Shao Kahn, and if you see him tossing his hammer at you, jump over it and repeat the process.
300: I used Scorpion for that one. First of all, if you lose a number of times and you press retry, you'll regenerate some health after each fight.
Goro: Spam backward jump and air teleport, like you did with Ermac against the Goros.
Kintaro: He is not particularly hard to fight against, once you figure out his attack patterns. Generally, stay away from him, wait for him to make his move, and counter attack him.When you see him jumping to stomp you, jump backwards and kick in order to counter him. If he throws fire ball at you, duck to avoid it, spear him, and do the (F+2, 1, 4) combo which kicks him back enough to keep a proper distance.
Mileena: In my opinion, the hardest fight of all. Once fight starts, keep blocking because she seems to have an attack pattern to beat you really hard when she gets close. But from a far distance, she becomes a little clumsy. So, get away from her, and spear her. Then, do the (F+2, 1, 4) combo to kick her back, and as soon as she gets up, spear her again. You should be very careful with your defence though, because if she starts hitting you, she won't stop before she's kicked your butt. You should try to avoid jumping as much as you can during the fight, because she can easily punish jumps.
Shao Kahn: This guy's a beast, until you start jumping over him. Once fight starts, hold block and let him make the first move. Then, jump over him (dashing forward if required in order to close the distance), and do the following combo: 1, 1, spear, 2, 1, 2. This combo, won't kick him back much, so that you can dash and jump over him as soon as he gets up. Fortunately, he'll do a wake up move, which will leave him vulnerable in the rear. By jumping over him and repeating the combo, you'll exploit this. If you find yourself too far away from him, wait for him to make his move, and teleport so that you close the distance, then repeat the combo.
This is not a bullet-proof strategy, but that's how i did it. If you can't, just practice and eventually you'll complete this challenge.
251: I guess Shao Kahn's your problem here, so find a good combo, and do the same thing as in the 300 challenge: Jump over him and combo, and repeat.
Good luck!