You will come to a series of drop platforms. These go down fairly slowly so when you see the one with the Star Coin underneath it just ride the platform down and jump off when you get the Star Coin.

In the next section where you have to keep jumping from platform to platform the Star Coin is on the far right under a small tiny platform. Wait until you are well below it to go for it so you have multiple platforms to jump to after you get it to avoid the lava.

If you have Fire it takes about 25 hits to put Bowser down. Otherwise you need to wait for him to do his super jump and then run underneath him.

The Star Coin is high in the second section. Stand on the ?-block to get Bowser to blow open the top area and its an easy grab.