Forget fire flower.
If you want to have a really easy time at the boss just bring along a mega mushroom. The first time I had to face the boss of that world (which should be the mole in the tank or at least that's the boss I'm referring to) I happened to have the Mega Mushroom in my reserve. So, I tapped on that item, grew giant and jumped on the tank, end of boss. Hilarious.
Strangely enough, when I was collecting the coins, I was playing that castle and I again had a mega mushroom in reserve. I wonered if I should try to play the boss normally but it was just to much fun to quickly waste him again. I have seince played it normally and the truth is, he's probably one of the more challenging bosses. But I'd have never have known thanks to Mega Mushroom.