Run close to his front side, so as to provoke him into jumping on you. Either run around him, or run between his legs as he does this, jump back on his back while he's down and perform a ground pound on the patch there.
The methods for doing so depend on your controller setup:
Standard Mode: B + R
Touch Mode: B + R
Dual Hand Mode: X + B
You must perform 3 ground pounds on his back to best him. He will kill you if he manages to crush you three times too however (or less if you're damaged).
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When that big guy wham's the ground, you have to get on him and ground-pound him on the back about 4 times and you beat him!
Let him fall, but not on you! When he tries to crush you, let him, but escape the right time. When lies on the ground, jump on his back and pound him. Do it 3 times.
First you jump up, then you press the "L" button.
Press B to jump, then R to do the ground pound. Any form of jumping, whether its normal, double or triple, you can still do a ground pound. Hope this helps!
It's when you jump up, then press down. If you are super Mario, you will break through bricks.