This is the Reverse Boss Order route used in the current world record (July 6th 2019). There is also an older route that this route is heavily based on, which you can find here: https://www.speedrun.com/smb/guide/48wyz

This category makes use of a glitch called bandage duplication, which allows you to access parts of the game’s memory in strange unintended ways. This guide will not explain why this route works as it does, but rather just what you need to do in order to execute it.

Route:

Beat 1-1 through 1-3.

Enter the warp zone in 1-5 and collect the bandage in the first level. On the second level you want to collect the bandage twice. In order to do this you need to press space quickly after reaching bandage girl and then press the replay level button. At this point you should have collected 3 bandages in this warpzone and may finish the warp zone normally.

From now until the end of forest, you should refrain from exiting the chapter or leaving to the world map unless told so. Dying on levels is not a problem.

Enter 1-11. Beat it once without the bandage and then 3 times with it. Continue all the way to 1-18 and collect the bandage in 1-13 on the way.

Here comes the most grindy part of the run: Collect the bandage in 1-18 34 times!! Nothing breaks if you collect it a few too many times, so if you lose track you can collect a few extra for safety. Once you’re done you can exit to map.

Enter 1-12x and beat it using Meat Boy or Dr. Fetus, whichever you prefer. Dr. Fetus can be found 4 steps to the left in the character selection menu. Exit to map. Beat 1-1x through 1-3x and exit to map. Beat the glitchzone.

Now the end and the Dr. Fetus boss should be unlocked. Beat Dr. Fetus. Rapture and Larry should also be unlocked. Beat Larry.

Enter the warp zone in 5-12. Similarly to the 1-5 warp zone, you want to collect 3 bandages. I would recommend taking the first one twice.

This is the second most grindy part of the run. Enter 5-16 and choose Naija as your character. Beat it once without a bandage and then collect 38 bandages in this level. Just like in forest, it's okay to collect extra ones and once again you may not exit to map during the process.

Now the first 4 chapters should be unlocked and each one requires you to beat 2 levels in that chapter in order to unlock the boss.

Beat 4-5 and 4-6, then beat Little Horn. Beat 3-5 and 3-6, then beat Brownie. Beat 2-1 and 2-2, then beat C.H.A.D. Beat 1-6 and 1-7, then beat Lil’ Slugger.

And that’s the run! Remember that the run ends as soon as Lil’ Slugger starts exploding and you lose control of Meat Boy.

Also, a major shoutout to -7 for the resources he’s made regarding bandage duplication and for the help he’s provided when I’ve wondered how things work :)