On this level, you'll spend most of your time on or near a special type of platform. This platform reacts to your weight by moving to the right. Always keep this in mind as you navigate the perils ahead.
As soon as you get on the platform, hit the ? block to grab a Super Leaf. Soon, two Buzzy Beetles will fall from above. Jump on one and send it flying into the other to take care of them. Soon after that, you'll see two rotating platforms below a Star Coin. Quickly jump onto the left and then the right one to go up and grab it. Up ahead, two more Buzzy Beetles will come flying out at you from above. You can jump to stop the platform and make sure they land in lava, or you can simply jump over them. Next, you will see a pair of thwomps, and you will have to be careful and jump up to avoid the platform from advancing past them underneath you. After that, you will reach the midpoint flag.
Grab a Super Leaf from the ? block to the right and get onto the next moving platform. From this point on, you will have to deal with fireballs that jump in and out of the lava. Hit the block in the middle of the first formation you see to get a springboard. You can then use it to jump high into the air and get the second Star Coin, which occasionally flies over to the right. Further right is a Red Ring, which is not particularly dangerous to try and get, but is not necessary either. When you see a red pipe up above, use the spring to go up into it to get the third and final star coin. When you reach the large doorway at the end of the tunnel, jump up to the far right to get a Super Leaf. When you're ready for the boss, go through the door.
Boss: Iggy Koopa[edit]
Iggy Koopa drives a chariot pulled by a large Chain-Chomp. It will jerkily advance into your general direction, and is invincible. Iggy himself will occasionally also shoot fireballs, but the Chain-Chomp is the primary threat. The secret to the fight is to jump on Iggy himself a total of three times. To do this, you may need to send the Chain-Chomp in the wrong direction. Try staying down low, luring it down there, and then jumping onto platforms above to get onto the chariot.
After being hit, or not being hit for a period of time, Iggy will retreat under or above the area. Then, sand will appear where he reemerges. Use this as an indicator of where to avoid being hit by the aggressive Chain-Chomp.
After three hits, Iggy will go down. You can then grab the key and advance to World 3.