All the levels in World 8 feature all the challenges, enemies, and problems associated with all the other worlds combined. It's big, long, and problematic, but, I have faith for those using this walkthrough.
This is the longest level in the game and you only have a time of 300. Make sure you don't lollygag as you could easily run out of time in this level.
After a Buzzy Beetle and a few Goombas, you'll come to a pipe.
Two Koopas here and a set of pits you can easily run over.
(Those Buzzy Beetles are actually Goombas if you are playing the normal difficulty). Grab the coin, and then jump over the "Goombas."
If you did it right, not only will a Buzzy Beetle not appear here...
... but you will also gain a one up for your troubles in an invisible block.
Keep on going; you will pass this pipe...
...and get to this pipe. (This pipe will take you to Coin Room B). (Also, those are Goombas).
A short pipe here, and, sometimes there will be a lot of Troopas here, and sometimes, there will only be three or four. Watch your step as you pass them.
"More Goombas." Another short pipe right before...
...this area. Going on top of the bricks instead of under them is the ideal way to get through. However, jumping from the right side of the tree will reveal an invisible block, and jumping from there will reveal 10 coins from the brick above.
You can either try and ignore this Paratroopa, jump on it repeatedly to get rid of it, or shoot it down with fireballs.
The Paratroopa at the very edge of the screen will almost always fall in a pit, so just patiently wait until it does before crossing.
After the pits, you'll come to these set of bricks. The brick in between the tree and fence has a star in it. Picking it up is very easy and very helpful.
With the star in place, you should be able to trailblaze through these Troopas,...
...jump over the pit,...
...beat the "Goombas",...
... carefully jump from pipe to pipe (better not to run),...
...take out the Buzzy Beetle and three more "Goombas,"...
...more Goombas, climb the stairs,...
..., and you should have enough time left to take out the last Buzzy Beetle.
As for this pit, it is easier cleared walking than running, as you have plenty of room to do either one. Try not to hesitate.
Another Troopa that you can ignore.
This one is easier than it looks. All you need to do is walk until you get to the last section of ground, jump, then jump again, all without stopping. I know it doesn't look like it, but you do have plenty of room.
Watch out for that Troopa on your way across.
Jump over those last set of Troopas.
Two more pipes to cross.
Now you've got this. For those who like to hesitate, be warned. Going one at a time is indeed possible, but very risky, and you're likely to fall. I would suggest jumping to the first one, then stop, and then jump all the others in one succession without stopping. If you do that, your chances of falling off are reduced.
Luckily, that's all there is to it. Other than the length, there is nothing too terrible about this level. Don't get me started on the later ones.
World 8-2, while being pretty bad, has one really good part about it.
Start off by avoiding this first Paratroopa.
Now, before you start to panic, the Lakitu will not be here the whole time, only just the beginning. Wait for the other Paratroopa to come down the steps. Most of the time, it will fall in a pit.
As soon as it's gone, run to the penultimate step of the first set of stairs, and wait for Lakitu to float to your right.
Now jump up to the next set; hopefully you can get to the top without getting hit by any of his Spinies.
Once you get to the top, Lakitu is really easy to kill. Wait for him to start floating your way, and then, when he is almost to you, jump and you should land on him, taking care of that pest once and for all.
Those Q Blocks have coins so grab them if you want and...
...continue onwards to this springboard.
As you can see, directly above the springboard is a one up mushroom. That Parakoopa will almost always fall in a pit, so wait until it's gone before running along the ground.
Two more Paratroopas. Jump on the first one multiple times to get rid of it, and then try and jump over the second one so you can catch up to the mushroom.
Jump on top of the bricks so the mushroom can come to you. An important note here: when you die, (and you probably will), you have to start over from the beginning. The good news is, this one up mushroom will keep respawning after each death, so as long as you are able to get this every time, you can retry this level as many times as you want until you are satisfied.
Jump from the bricks to the Cannon.
Go from the Cannon to the other Cannon.
Go from the Cannon to the bricks. The brick on the right has a power up, and, even though it's dangerous, I highly suggest going for it as it will be very helpful.
Moving onwards, we go from bricks to Cannon to bricks...
...to Cannon to Cannon on bricks. Staying off the ground as much as possible will help you survive.
Jump to the other Cannon.
Now jump to the pipe.
Sometimes there will be a Paratroopa here, so ignore it or get rid of it and then get to this second pipe.
This jump right here can cause a lot of problems. One way to do it is to run and jump over the pipe on the left, land on one of the small pieces of land, and jump to the other side. I find it more successful get on that pipe, normal jump over to the far small piece of land, and then quickly do a run jump to get over the rest. You are more likely to land on the small strip if normal jump rather than run jump, and once you are on it, you can, usually, activate a run jump fairly simply. (Hesitation is simply not an option here).
That was probalby the worst part, so get over these pipes here...
...being careful to avoid or take care of the Paratroopas while you are at it,...
...and then jump on to the Cannon, and, from there, to the piece of land on the other side.
The "Goombas" here are easily avoidable, so pass them by.
Here, you have your last set of Cannons along with your last Paratroopa. If you stand on the Cannons and get a good enough angle the Paratroopa will start to come down the steps. Wait for it to get close, and then jump over it and onto the stairs.
Bypass any stray Bullet Bills that come your way, and carefully make your way up the stairs...
(If you have lost your power up, I highly suggest killing yourself, and making sure you complete the level with it. Trust me, you'll want to be at least big on the next level).
...to the flag.
Here is a cannon you can easily get by...
...as well as a Paratroopa and another Cannon, both of which you can easily get by.
After that, you'll get to a pipe here. Jump on and over it and then get a little bit further in before retreating back on to the pipe.
The nice thing about these guys is that all the usual strategies work. Wait for both of them to be off the top or bottom sections, and then run as fast as you can.
As soon as you get to these stairs, STOP! Make your way to the second step down, almost reaching the third step.
If you've done it right, both Hammer Bros. will be off screen and out of harms way.
The brick right above Mario can be used to get a power up. Power ups, like I said, are very helpful, and are few and far inbetween, so get them while you can. (If you had retained your first powered up form, you could've gotten a Fire power here, something EXTREMELY helpful in this level and the next).
Climb down the stairs, and you'll face another Cannon...
...and Paratroopa, both of which are easily manageable.
Follow the path to this next section of Hammer Bros. If you have fire power, take them out! Otherwise, use the same strategies to get past them. Unfortunately, the power up is right around above where the Hammer Bros. on screen is, and is very risky to get, so I wouldn't go for it, unless you have confidence in yourself.
Jump off when the Piranha Plant is gone, and be careful not to get hit by that Troopa.
This Troopa is actually a godsend for those who do not have fire power. Get the Troopa in its shell, and propell it across the screen.
You'll come across not just one...
...but two Hammer Bros.! Since there are no safety platforms to use to avoid them, simply run up to them as fast as you can and immediately jump over them without stopping. Chances are, by the time they get to attacking you, you'll already be out of their attack radius.
Another pipe here, with, yet, another Hammer Bros. waiting close by.
Two more Hammer Bros., making a total of four in a row, two on each side of the pipe. Either run, jump, and avoid them, or take them out with fire power.
If you can get him out of your line of sight, you'll notice a suspicious part of the castle wall. That wall is actually a brick that contains 10 coins.
Now, as for this set of "stairs", the same strategy applies to this set as it did to the set in World 8-1. Jump onto the first one, stop, then proceed to jump from block to block without stopping.
Hopefully, you got out of that unscathed, fire power still attached, and are ready for the final castle.
Welcome, to World 8-4.
To start off, go to the very bottom step, and then normal jump. Mario should just barely be able to make it.
You'll come up to a pipe that you can easily jump over...
...proceed down a very long hallway with nothing of interest...
...jump over another pipe and onto some Goombas. If you want to play it safe simply wait for them to be out of your way before you cross over them.
After that, you'll come to another pit.
While you technically can jump over that, it is very risky. Far better be it for you to take this platform that you can barely see over to the other side.
The trick of this castle is, if you just keep on going after this pipe, you'll wind up, back at the beginning. Instead, take this pipe down to the second part.
Here, you'll have another pipe, as well as some Buzzy Beetles to ignore.
Get past them and that pipe to reach another pipe with a Paratroopa right before it. Ignore it or take it out, and then proceed onwards.
Jump over the pipe and over the pit to encounter two more Paratroopas.
If you are still fully power up, take them out with fire power, otherwise, carefully avoid getting stepped on by going under the first one, then falling back slightly, then going under the second one.
After that, you'll find an invisible block here. This pipe will take you to the third section.
This section doesn't seem like much right now; it just has another pipe with no other enemies.
Unfortunately, this section has Flying Cheep-Cheeps coming every which way, so try to avoid standing still, and get to the end as fast as possible. These guys fly farther and higher, so don't be surprised if they fly up through the ceiling, only to reappear from the ceiling back towards the ground.
Jump the pit...
...and proceed down that pipe for the fourth area.
Now here's something new, a pipe with an underwater background. Due to the precense of water, it is impossible to see the Piranha Plant that appears right after you exit the pipe, so just be warned in case you're lolligagging.
Fire underwater? Whatever the case may be, stay close to the top as much as possible here...
...until you get to this opening.
Wait for the firebar to pass on by before proceeding.
Here you have two more Bloopers. If you still have fire power, you can try to take them out, but I think it's easier to just ignore them and get to the end of this part.
The best way to do that is to get between them...
...float all the way to the top...
...wait in this corner for the firebar to pass you by, and then go into the pipe to part 5. If you are still powered up, the firebar will hit you, so try to stay as low as you can before entering the pipe.
Part 5 is the last part,...
...and who better to be the last line of defense before Bowser than another Hammer Bros. If you still have fire power here, take him out!
Otherwise, get as close as you can without touching him, and, surprisingly enough, none of his hammers will hit you.
Wait for a gap in his throwing before you make your move against him. Unfortunately, his pattern is irregular, so wait for that split second where he is taking longer than usual to throw and jump on him. He usually gives a good amount of time between each set of throws.
However, don't think you can just rush on in there, as there is a Podoboo here. It also jumps at irregular intervals, so as soon as it is below the ledge, jump!
There he is, Bowser!
This guy has fire balls, hammers, and Podoboos all protecting him. If you still have fire power, hit him with everything you got from a safe distance. If not, your best hope is to get close to him, and, as soon as he jumps, run under him to get the switch. Jumping over him is almost impossible now as he jumps a lot higher now.
If you're lucky enough to get past all of that, you can hit the switch, send him falling to his death,...
...and finally meet Princess Toadstool!
As a reward for beating the game, you are now able to play through the game again, with some variations. All the Goombas turn into Buzzy Beetles, and the enemies move faster. Everything else is the same.
When you get back to the menu screen, it'll now show your top score on the bottom.