IMO, Jade is the hardest secret fight to get if only because getting double flawless on shang is pretty damn hard. Even on beginner he manages to nail you with cheap shots. He has an answer to everything you could possibly do. The closest i've gotten so far is with Sektor, but I got too frustrated to keep trying after a while.
Everyone keeps saying you can't lose a round - that's wrong. You can't lose a match, but rounds don't matter. I know this because I had 2 lost rounds and still got Jade.
Hmm... wish I knew that before instead of thinking I had to go 2 for 2 flawless against Shang. That leaves a lot more room for error if you can just lose the round where he hits you instead of starting over. Oh well.
Only one more secret fight to go; still haven't seen Noob pop up in the temple.