Here's how I did it:
First, put the easiest settings in the options (CPU Level to 1, P1 Defense Level to 8, P2 Defense Level to 1 and P1 Power Gauge to Max).
Select the team of Haohmaru and Genjyuro. Then, in survival, spam their foward + Y slash attacks on your enemies. Pretty much everyone will fall for it AND it really will DESTROY your opponents (because of the weak CPU settings). If you get in trouble, don't forget to switch partners to recover health as much as possible.
You should get 15 wins pretty easily. Personally, I got 32 wins by doing just that. Once you get to 15 wins, the achievement will automatically unlock.