-Go to Dojo
-Select Character With "Team Turtle Power KO" Unlocked
(In order to use the TKPO you must be in a 10+ Hit Combo
then hit RT+X around the other three turtles)
-Select "Team Turtle Power KO"
-Now that training has begun move the other three turtles facing one dummy (i used the left guy) leaving an opening for you.
-Try to build your Combo up on the other two dummies as much as possible using x,x pause x,x so you don't knock the dummies down right away
-Once your Combo is at 10x and you feel you are in position hit RT+X
When it popped for me I built an 8x Combo on the open two dummies, moved into the third, hit him twice then RT+X'ed it.
Also the three turtles were facing the dummy not away..
As MILLER TIME has mentioned if you have "TPKO Threshold" unlocked you will only need an 8x or 5x Combo depending on it's level thus making this Achievement even easier... :)
See MILLER TIME's guide for the video walkthrough...
May take a few tries... Good Luck!