This is pretty easy once you learn the timing of it. First enter Practice mode and change the settings so that P-Power is max (default is S-Power but this move only works with P-Power) and then practice the motions, assuming you're facing right:
F, DF, D, DB, B, F+X
Basically you roll your thumb in a half circle backwards on the D-Pad or analog stick, pause for just a moment, then hit forward and X at the same time, the screen will flash and Deadly Wave Neo will begin. You must now input the following:
X, X, A, A, Y, Y, B, B, D, DB, B, Y
Rock will continue on his combo and it will hit 15 times if done correctly. The buttons just mentioned will have to be mashed out as it's very quick, and it has to be one fluid motion otherwise the combo will end. The opponent must also be on the ground when you hit them otherwise Rock will go under them as he moves forward, and they need enough life to absorb the whole combo, as it ends if they are KOed.
It has to be done in Story mode so I suggest turning down CPU AI to 1, P1 and CPU defense up to 8 (so that you can both take a lot of hits) and P-Power to Max, and provided you've practised the move and can do it comfortably, you'll do it first or second time as the computer doesn't put up much of a fight on the lowest difficulty.
Altogether:
F, DF, D, DB, B, F+X then mash out X, X, A, A, Y, Y, B, B, D, DB, B, Y and the achievement will unlock if done correctly.