Nightwolf is cheap in that challenge. All you have to do is spamming lightning with Goro and Kintaro - as they don't jump and lightning is unblockable, it's an easy win. Also, if they happen to send you a projectile, use Nightwolf's absorb to gain life. Mileena is the worst, and Shao Khan is pretty easy with Nightwolf.
I used Stryker and it only took 4 tries. After pwning the 3 Shao Kahns with him, I knew if I could at least make it to him, I'd win. It's exactly what happened. First time I saw him, is the time I beat that challenge. So glad that tower is done lol.