This is the third boss fight in the game. He will throw 3 small rocks at you and then a big rock. Spinning the big rock once it has settled on the ground will fire it back at him. Whilst this is happening, TNT will fall. So not only do you have to dodge the rocks, but also the TNT.
What I did was run back and forward for the first few waves and then as more TNT fell, I would sit in a corner and just before he throws a rock, I would jump away and then land back in the corner.
Note that hitting a mine kart with the big rock will not do any damage, so time the spin back appropriately.