I think I can help here, Crash Man.
First of all: I happen to have all of chapter 5 dark world online here: http://www.youtube.com/view_play_list?p=654885301C379B21
(Bandages not included, I didnīt have them yet when I recorded those)
Now, some notes:
5-4x: The Kid is not the only character that can pass through to the middle, Naija can as well... and Meat Boy!
Jump to the right in the beginning, and wall jump up once, then to the left. You will either fall (bad, try again ^^), get stuck on the little edge (jump and push left to get in) or you pass through the gap immediately.
Personnally, I found it easier to go back to the first room to get upwards, but thatīs your choice.
5-7x: Hard level, I donīt know if I ever finished it with Meat Boy, Naija is good (you are on steam, or XBox?)
5-8x: If you have Runman: Use him, makes the beginning part easier due to his speed, and he feels less "sticky".
On the part where there is a gap on the left, and then a gap on the far right (before the moving saws), you have to make a little hop. This means that you should not jump before that part, or you probably wonīt make it.
The part with the horizontally moving saws is easy without bandage, just stand on the left and move left/right in very small steps, make a short hop on the last row.
I can upload a vid of how to get the bandage, itīs hard to describe. (Already had it recorded, just have to encode it, if you want it, just say so. ^^)
The part with the vertically moving saws: wait around and stick on the wall, there will be a gap where you can jump over the outermost saw and then move to the side again.
5-10x: Not too hard with Meat Boy, start by landing left of the pit, then alternatively jump left and right of the pit, collecting the key in mid air. Youīll have to find a rhythm, so that the zombie guys run into the pit. When you get the last key, all of them except the 2 on the left side will be dead, and you can just jump over those with a maximum height jump.
5-14x: The par time on this one is very long, so just take your time and lure the carrion kings away, and then get away from them fast, so they go to sleep. You should have none of them chasing you when you climb to the key or back up to the start.
5-16x: This one is tricky. Start by running left, so that you cling on to the debris part, and immediately wall jump to the right, towards the gravity orb. You should land somewhere on the thing where bandage girl is. From there you can easily jump to the pillar below.
For the part with the falling saw you have to find a rhythm again. Keep pressing the direction button towards the wall you are on, and jump when the saws get near, you should pass right between them.
After that part you almost have it, as you can skip the right side of the level with a little wall-jump action.
Jump from the wall above the exit to the flat platform above.
5-18x: This level consists of 2 jumps and in between them you just have to jump up and down in the rhythm of the rocket launchers.
Watch the video, itīs hard to explain. :-)