This is, arguably, the hardest level in the game. It is strongly recommended to bring a Tanooki to this level (and a spare one as well). If you lose it, just quit. There aren't any power-ups in this level. This level is only accessible when you have completed all levels with Mario and Luigi, getting the golden flag on each one (you only have to get the golden flag with one of Mario or Luigi, not with both). This is also the only full level without Star Medals.
First area[edit]
You start on a big block in space with some Toads on (they won't give you anything, sorry). A few blocks further, you confront two Hammer Bros. Kill these if you wish, or just avoid them. Even further on, you'll meet a Boomerang Bro. You can just duck under his boomerangs (or jump on them), but the situation here is the same as with the Hammer Bros. Also, if you accidentally miss any of the blocks, if you aren't a Tanooki, commit suicide. If you do, wall-jump and hover. Next, you'll find a big block with a warp box on. Jump in.
Second area[edit]
You are now on another block with a Toad. None of the Toads will give you anything, so ignore him. The next block holds a Goomba. Do whatever you want with him (except get hit). Two blocks later, there's another wooden block. Choose what you do next carefully: Either you can Ground Pound and kill the Goomba on it and jump (maybe Rolling Long Jump) to the other side or, the harder option, hold the jump button when you land on the Goomba and propel yourself to the other side. On the next block, there's a larger block with a line of smaller blocks protruding from it. Kill the Goomba. Jump over and Ground Pound on the two-block line. After that, there's a dotted line of tiny blocks. If you haven't got a Tanooki, grab your spare one now while it's easy. Jump over and ground pound on the blocks as you go along. At the end, there's a longer gap and, on the other side, a safer area. It looks possible with a Long Jump or Rolling Long Jump, but looks can be deceiving. This is why you brought the Tanooki. Jump and hover over. You can now safely move everywhere except towards the camera. After a few blocks, you discover a tiny gap. On the other side, the first block is slightly higher. Just jump over. You can now choose how to go. Either get on the slightly-higher block and wall-jump-hover your way to the top of the blocks and hover to the other side or try and get to the other side by bouncing on the Goombas. It is recommended to take the former because if you take the latter, the tiniest bit of upper diagonal-ness can force you to wall-jump off the block on the other end. You will now meet a Block Snake. If you took the former route, try not to jump on it as the Tanooki will make the next bit easier. Kill the block snake by hitting all the blocks in it (except the front and back), then backwards-somersault onto the next blocks. They are arranged in a staircase-like formation with two steps. The top "step" is two blocks long. Hover over to the next bit and ground-pound onto the two-block line. Try to make sure that you take the bee with it. Hover and ground pound to the next single block and to the big wooden one. The next tiny block has a Toad on it. For once, he actually has a use: hover over to him, press jump when you hit his head and hover to the next line of blocks. Now just walk over to the pipe.
Boom Boom and Pom Pom[edit]
Yep, these guys are back. And you have to fight them both at once, like in World 7-Airship. Boom Boom is on the bottom while Pom Pom is on an extended part of the floor. Luckily, if you've still got your Tanooki, you can kill them quickly by spinning them and avoiding the shells. If you kill Pom Pom, her bit will lower down to Boom Boom's level. This does not go the other way round. When they're both dead, go down the pipe into the next area. If you got hit during the fight, don't worry, you don't really need the Tanooki any more.
Third area[edit]
You arrive on a block with, next to it, an Arrow Lift. Obviously, you need to go away from the edge. Avoid the Fuzzies. You will fall off an edge. Either jump off while it's falling or when it's gone down the two extra slopes, quickly jump to the next Arrow Lift. Go away from the edge and try not to get hit by the Tanooki Bob-ombs. Bop them with your head if necessary. Avoid the fire bars. Jump over to the next platform and jump into the warp box.
Fourth area[edit]
Run/jump along the falling logs, jumping over the fire bars along the way. Jump onto the first solid platform without spikes that you see and jump into the warp box.
Fifth area[edit]
Giant Cosmic Mario's back! Roll (or run if you're small) under the first left column of fire bars, avoiding the Chain Chomp and the spiky balls. Once you're under the first one, you don't have to go under any more. Jump when the big block things appear and run past them. Get into the warp box behind the second one.
Sixth area[edit]
Cosmic Mario is no longer on your tail. Stand on the green switch and get onto the platform. It is not recommended to try and take the path along the green platforms. The message "THANKYOU!!" will appear and when the yellow platform stops at the dot of the second exclamation mark, jump into the warp box.
Seventh area[edit]
This place is full of Toads. Hop up the blocks and grab the flag (if you want the top, don't Long Jump, just jump as far as you can).
Congratulations, you have finished Super Mario 3D Land! (Although you need to play this again with the other character to really have finished it 100%...)