Complete the indicated task in Arcade mode to unlock the corresponding battle.

Secret "Jade" battle: Get a double flawless victory and perform a fatality on Shang Tsung when battling against him.
Secret "Noob Saibot" battle: When you see Noob in "The Temple" stage's background, win that battle without using [Block].
Secret "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.
Secret "Smoke" battle: On "The Living Forest" stage, wait until Smoke appears behind one of the trees then immediately press Down + Back repeatedly.