For this trophy you'll want to Ring-Out KO your opponent during battle. And it's best to do this in VS battle with a buddy or 2nd controller.
To perform a Ring-Out KO you'll need to select a particular stage,one that may have a ledge or cliff that would provide you with a Ring-Out feature. You'd then have to break the invisible barrier by launching your target into it twice for it to shatter. then you'll have to launch your opponent 1 last time, into the exposed opening to send them flying for an automatic KO victory!
The following stages have the Ring-Out feature:
Leaf Village (Reconstruction)
Grassy Waves Prairie (Night)
Hidden Rain Village (Upper)
Final Valley (Rain)
Great Snowfield of Iron
Storm Cloud Ravine
NOTE: Rasengan users are ideal for this trophy.