Perform a 10-hit combo with any fighter.
Actually, this one is pretty easy. Sonya's X-Ray attack is doing a total of 11 hits and this trophy or achievement will be yours within a couple seconds. You can do this in any game mode, but practice mode offers the best chance. Anyway, you can also get this automatically during the first couple missions of the Challenge tower, in a mission where you need to perform Sonya's X-ray.