Managed to grab Demon Boy and I have to say that I thought it was considerably easier than Blood Clot Boy. When you pass 4-4x and 4-5x it's like 10 levels in a row that you shouldn't fail. The last five are easy to make mistakes on, but the amount of levels that can be gamed with the Kid and Viridian is very high. I did restart on 5-15x a bunch of times even though I probably didn't need to, but it was towards the end of my run and if you're not sure the projectiles will hit where you want to it's better to play it safe IMO.
"Just" four achievements left now
edit: I did the run in this order in case anyone still needs tips on it:
4-16x - it's straight forward but the last jump can get messed up if you hit your head on the little wall sticking out from the ceiling.
4-17x - no trix on this one either, just need to be extremely precise with meat boy.
4-18x - I used Viridian on this one. His jump is floaty though so it's easy to die on the first jumps. I knew if I had it late I would screw it up.
4-19x - Another Viridian level. You just stand on the roof and wait for the elevator. It's easy to die on the way down though because Viridian is so sluggish.
4-20x - I die from weird bugs on this level more than I do the actual obstacles. Ugh. If you get into the second room on time you should be fine, it's just the first jump that's tricky. I immediately run to the edge of the moving platform, then do a standing jump to the middle wall and if you walljump off that you can do a full height jump, just holding right, and never hit the top saw. It's the most consistent way I found to do it.
4-4x - I didn't think this level was very hard but I always died on it so I moved it up.
4-5x - My way of doing this level consistently was to find the right spot to jump from the right platform to the one moving vertically. If you jump (running) about one block before the edge and then immediately walljump to the right you should go through every time. It's still a very precise jump.
4-6x - Use the Kid, simple.
4-7x - Nothing special about this one either. The timing on the saws allows you to pass them smoothly.
4-8x - I thought this one would be super hard, but I worked out the jump timings for just racing from the start. If you just go as fast as you can it all lines up quite nicely, but it was the level I practiced the most.
4-9x - Another level for the Kid. Just double jump vertically at the start.
4-10x - I used Viridian for this one, but the jumps still feel really scary. He controls so poorly :X
4-11x, 4-12x, 4-13x - all of these are fairly straight forward with Viridian. The second key on 4-13x can be tricky, but fall down to grab the one on the right, then run left and flip when you're on the second dissolving block.
4-14x - I really like this level. When you're on the upper level do a high jump over the last "bump" but land running and let go of the movement keys when you hit the air, you will hit a block and fall straight down, perfectly, then you can just make tiny hops over the blocks on the bottom.
4-15x - There are probably lots of ways you can do this level, but I start by jumping on the box bandage girl is in, jumping over the projectile, jump to the key, then do another jump there just to ensure that the third projectile hits that wall at the top. Then you go over to the right and make sure your jump height allows you to pick up the last key as you land.
4-1x - I used Viridian for this one too. I flip down between the two sawblades in the middle. There's a rock on the roof you can use to gauge your position.
4-2x - For this one I used Naija, I did three air-dashes. First time to get over the saw to the small shaft you have to fall down, then as the platform moves forward do a very short jump to the wall on the left and walljump off of it. When you're about one block away from the place where you could land do another dash, you will hit the wall on the right. Do a tiny wall jump just to reduce your speed and do an air-dash under the saw and you will land between them both. It looks super dangerous but it's a very consistent method, I'd say it was my best map.
4-3x - Viridan here as well, 8th map with him I think. I always get freaked out with pressing escape fast enough. If you don't move in the air you will stop halfway on the way up, but you still have time and it's safer that way. Good map to end on IMO.
Also, just a random tip that I think a lot of people don't know. If you don't hold any movement keys when you land from a jump you stop, completely. You don't move a pixel. This makes levels like 2-10x very easy. Just run off the side then let go of any movement key, you will land on the block. Jump back and do the same thing with the other side. Then stand close to the edge and do the same thing with the block in the middle. I saw people doing that level all kinds of ways, but it's actually the easiest level in world 2.