In World 4-3, make your way over to the second group of wood blocks. There should be three brick blocks above you, one containing a 1-Up. Walk over a little more and you'll see a Spiny walking upside-down on some bricks, and a Buzzy Beetle walking on top of the same bricks. When you see this, don't get too close to the Spiny, or it'll drop down. Instead, walk slowly over and stand under the first block of that platform. When the Buzzy Beetle is above you, jump so you stun him as you hit the block. The Spiny will die, and you didn't even touch it!