I'm pretty sure you need at least 3 fatalities on other characters
Jade secret battle:
3 flawless victories + the double flawless on Shang. (5 in all)
Finish everyone with a fatality. (only need 5 though)
Finish Shang with a fatality. (required)
You cannot lose a round on anyone. (required)
After you finish off Shang Tsung with the 2x Flawless and Fatality, automatic Jade fight.