Ok here is what I do.
1. Move forward to one side of the approaching topman (it will sail past you) and pick up the item box, brake and turn on the spot to face the topman and mushroom into it.
2. Reverse about 1/4 around the curve while the 2nd topman appears.
3. When you are facing the topman use the second mushroom to boost into it.
4. Reverse around to face the centre, and time the last mushroom so you go through a second item box and into the furthest of the two topmans.
5. Then use the first of your second set of mushrooms to boost that topman off the edge.
Finally you should have 2 mushrooms left to get the final topman, often if you have timed parts 4+5 correctly and got the right angle, it will have spun in front of you just after No.5, so you can hit it straight away.
http://www.youtube.com/watch?v=aNBIrOoV0sI
