This is well hidden. Once you enter level 30 'Rhythm' and reach the water, look left. You'll see a platform with a vine leading up, and the platform gets flooded every time the water rises with the beat. If you look precisely, there's something moving in the water: an invisible boat.
Continue on your way through the level and when you reach the first set of floating turtles, you will notice that the timing is off and you will always end on the one turtle that won't let you reach the invisible one. Use the turtle to get to the other side as if you had never noticed anything suspicious, then turn around and jump on the other one. Make sure that you have activated the Tinker Flower, in case something goes wrong and you deplete your health points, otherwise you'll start back at the beginning of the level. Now you should be able to hop on the invisible one. The next step is tricky. Pressing R2 isn't enough to get off the turtle and onto the platform. Keep pressing R2 and hit R1 quickly to get off, then dash towards the vine (still keeping R2 pressed) before the water returns. Follow the path further upwards. The trophy unlocks once you reach the top. Drop off to the right side to continue with the level.