Well I missed it when I looked. There's a lot to take in this time around.
edit: anyway it's inconsistant at best. One challenge says "high kick then fireball" but in reality it's high kick focus dash attack cancel plus fireball. Another one on Ryu actually tells you focus attack cancel in the description. Yet both require it and only one says it. I'm assuming as I do more of them I would see this kind of thing more.
I managed to beat the first cancel challenge with Chun Li and the second one was crouching punch, focus dash cancel and then spinning bird kick. Problem is you have to CHARGE spinning bird kick and the computer doesn't want to give it to you unless you could somehow magically do a SBK instantly without charging.
It should be cut and dried. If it requries a focus attack dash cancel it should SAY that. It's hard enough to do this crap even if you know exactly what to do. The internet is full of people complaining about this same thing and they can't figure it out. The people who know are not really sharing that information with the exception of one post I found. If you know how to do it or tips to do it better why not share it with us instead of guarding it like a secret.
I've played capcom side scrolling fighters since Street Fighter 1 and nearly mastered games like Marvel vs. Capcom 2 and hands down these challenges are the stupidest cheesiest things I've ever seen. My hand is still soar from trying to do them last night... I couldn't keep this up or I won't be able to play at all through all the pain. At the same time if you don't, more or less, master everything in the game you aren't going to get half the achievements. I could probably name about 5 achievements all tied to finishing challenge mode completely. I'm not complaining about time trials or survival mode, that's cut and dried so no complaints.