Hidden "Jade" battle - Get a double flawless victory and perform a fatality on Shang Tsung when battling against him.
Hidden "Noob Saibot" battle - When you see Noob in "The Temple" stage's background, win that battle without using [Block].
Hidden "Reptile" battle - On "The Pit 2 (Night)" stage, wait until a shadowy figure flies across the moon, then get a double flawless victory and perform a stage fatality.
Hidden "Smoke" battle - On "The Living Forest" stage, wait until Smoke appears behind one of the trees then immediately press Down + Away repeatedly.