Ok I finally beat the Challenge ladder today. When I got to 300 and realized that they let you select your fighter I thought "Oh man this is going to be a endurance match of all 28 fighters or something."
And then I play...
You fight Goro (Stupid and Slow), Kintaro (Poses every 5 seconds),Mileena (Suprisingly More difficult than the bosses) and Shao Khan (Ok he's hard-ish, but we've faced three Shao Khans in a row at this point).... that's it. Well there are a few fight modifiers... all negative towards you, but for boss fights they are a non issue as you won't be blocking or anything anyway.
I've heard all of this complaining and moaning on the Internet about people needed to scrounge 50,000 coins to skip them and THAT is what people couldn't defeat??? Sheesh! This generation of gamers... I just don't know.
Btw... in case you are having trouble the easiest person to beat it with is Noob Saibot. His teleport slam works on all the bosses. Use it as they are firing something at you and you'll get them. As you "roll off" of the boss after the slam, immediately teleport again once you are upright. You can get them in an infinate loop. The computer can't even start adding fight modifiers until you miss! For mileena just beat here the fegular way, they've increased her speed by about 25% but that's it.
I beat them on the second or third try... not hard at all.

Hopefully we'll get some challenge ladder dlc, as that was my favorite bit aside from story mode.