The obvious method is to find a player with a 15 - 19 win streak in DOA Online Mode and defeat them.
Alternately, you can easily unlock this achievement with two controllers.
1) Change VS settings to 1 round win, and lowest health (speeds things up a bit)
2) Start an offline VS match with both controllers (do not sign in on your 2nd controller)
3) Use the 2nd controller to beat your 1st controller (the one signed into your account) 15 times.
4) On the 16th match, use your 1st controller to beat the 2nd controller.
5) Join a Quick Match in DOA Online Mode. If there is one in session, you can just watch it.
6) As soon as the match ends, if you've done everything correctly, the achievement should unlock.