Luigi's Purple Coins

So, my 7yo got Super Mario Galaxy for his birthday a few months ago.  Our MO has been he'll play a level a few times, and if he can't beat it, he'll pass the controller over to me.  So we beat the game, and then there is a bonus if you get all 120 stars.  We have 80 something right now, and we're trying to pass "Luigi's Purple Coins".  I can't beat that one yet; it's the only one I haven't been able to beat, and I've had like 20 tries at it.