With Yoshi, go to Bob-Omb battlefield. Shoot yourself out of the cannon near the starting point. Aim over to the areas on the left. Go to the orange bricks, and jump on top of the one with the mushroom inside of it. You should have a bob-omb chasing you, and he cant hurt you when you're on top of the box. He will blow up, and the box will open. You won't get hurt, and you can brab the Big Mushroom.