This world is possibly the most difficult in Super Mario Bros. 2—World 5-1. You must cross a waterfall that seems endless. You can do it by stepping on the logs and Trouters. One flase move and you're gone. Concentrate or you're in the drink.
Map A
This is easy part. You start out encountering a Shyguy riding an Ostro. It's a good idea to pick up the Shyguy and bring him along with you so you can take out the flame shooting Panser up ahead. Then drop down to the level below and enter the door to start the real action.
Map B
Part 1
(1) Sub-space 1. You can get a potion at the entrance of map B with no trouble, but if you're looking for the glitter of gold coins, you'll have to enter Sub-space here. In fact, since there are no other coin grabbing opportunities, you might as well exit back to map A and return again to repeat the process.
(2) Jump on Trouter again and again. You can't get over to the other side without jumping on the Trouter's heads three times in a row. This is the toughest part. Practice until you learn the best timing, and don't give up. The Princess can be the most forgiving when it comes to pulling this trick off.
Part 2
(3) Sub-space 2. The clump of grass on the left is a 1-Up. The one on the right is a Potion. If you enter Sub-space here, you'll find a Mushroom on the lower rock on the right. You've cleared half of map B already!
(4) Sub-space 3. Where! You probably had a rough time getting here, but you can't get the Mushroom the ordinary way this time. Make a big jump to the right of the last log and drop down the chute on the right in order to grab the Mushroom Block. Then use the Potion. This can be tricky to pull off, and if you miss the jump to the ceiling then it's hard to gain a second chance.
Map C
(5) Fight with a Mushroom Block! Here's Birdo again, and this one only shoots fireballs. Birdo fires three fireballs in a row, jumps, then repeats the pattern with another three fireballs. Your only weapon is the Mushroom Block to the right. Fight cautiously—it's more important now than ever.