World 5-1 is possibly the most difficult in the original Super Mario Bros. 2. It is significantly easier in Super Mario Advance. You must cross a waterfall that seems endless. You can do it by stepping on the logs and Trouters. One false move and you're gone. Concentrate or you're in the drink.
The 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 a level on the right side of the screen, where the vegetables can be seen, and make your way back left. Enter the door to start the real action.
In, use the Shyguy to kill the Ostro to get a combo (points and heart power-up), then dig up the giant turnip to take out the Panser. Dig up three of the four remaining vegetables (use a power jump to get back up to the top platform) and take them down by the door and use them to pop the bubble surrounding the 1-Up. Grab the 1-Up, then enter the door.
(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.
In, although the grass will respond, any coin picked up will be replaced by a vegetable while in sub-space. Also, the second grass patch of the lower five has a giant red Shyguy. You can use this guy to give you unlimited health. Keep him along for the ride; if you jump on a Trouter as it is rising, so you can get a boost, but you take damage, throw this giant Shyguy onto a platform, get the health that appears from him, then pick him up again. To get to point 2 and get both coins, use basic hops and hovers to get from platform to platform and get the top coin. Then jump over to the right platform at point 2. Then, wait for the previous log to come down the waterfall, jump on it, get the coin, and hover back.
(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.
In, there are only two trouters and the gap is smaller. Hop onto the first one, then jump and float across to safety.
Before you even reach the first original mushroom, in, you are given a rare opportunity to discover one of the many newly added mushrooms to the game. Grab the potion found at location 1, and carry it with you all the way to location 2. Drop it down here, and enter Sub-space to increase your vitality with this new mushroom before you even find any of the original mushrooms.
(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 (in, the Mushroom is high in the air, so you'll have to wait for it to fall). You've cleared half of map B already!
(4) Sub-space 3. Whew! You probably had a rough time getting here, but you can't get the Mushroom if you went straight to where the grass and exit door are. Instead, hop to the second log, then power up a jump. Right when it is ready, make a big jump to the right so that you land above the screen on top of the rock. Run and jump over to the right and drop down the right chute/hole to land on top of the Mushroom block. Grab the Mushroom Block and drop down. Throw it out of the way. Then use the Potion below that right chute. This can be tricky to pull off. If you miss the jump to the ceiling, then it's hard to gain a second chance.
(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.
After defeating Birdo, grab the Crystal Ball and head through the hawkmouth gate.