Dojo Advanced Lesson 27 provides information about how to get Shadow Counters. You need to hit medium punch and kick (Y+B on default controls) while blocking during an enemy combo. Shadow Counters will only land under certain conditions, which the dojo lesson covers. (They should be executed during the last hit of the enemy combo, and need to "grab" the opponent. If you mistime the counter, you'll leave yourself exposed for further attacks.)
I got this by playing against a Noob CPU Versus match. It'll take some time to get the timing down, but the Noob CPU likes using heavies, so if you miss the block, combo-breaking out should be easy enough. Also helps to set match timer to infinite. All three counters need to be done in one match.
The challenge should unlock once you get to the match results screen.