I initially thought this meant the shell take downs that you can do but apparently that's not it. Although the name shell shock for that move would be all the more appropriate.
In each turtle's skill tree under Attacks and Finishers and third row down, and center, is the move Shell Shock. You will need to spend at least 3 skill points in the combat tree to unlock shell shock. (There may be a level requirement. I'm sorry, I didn't think to look for one earlier but if someone knows what it is I'll edit it in and give you credit)
To use this move the enemy has to be on the ground and you must have either A) your combo, aka TPKO mode, ready to be used or B) all your special bars filled. If the above criteria are met Shell Shock is performed by holding RT + B.
^^^ This is a pain during normal play.
Best way to do this is to, like all the other grindy achievements, go to the Dojo. There select the move list and choose the turtle you have unlocked shell shock for. I recommend choosing a turtle who you have NOT completed the 100 KOs for as you can kill the majority of two birds with one stone here. Then simply repeat the practice round of Shell Shock with that turtle until it pops.