Since there hasn't been any actual talk of Iron Boy runs for over a page on this topic, let me share some tips for Dr. Fetus Boy:
6-1X (Detox): The only really tricky part of this level is the very first jump off the block of rotating buzzsaws. If you get stuck at that part, remember that sliding onto the wall, then not moving at all while you fall down should protect you from the hitboxes of the buzzsaws. Then, you can time a jump over the saws from there. Everything else should be cake.
6-2X (Ghost Tomb): Like SA667 suggested a couple of posts back, exit to the map immediately if you fall into the pit. This one is extremely simple, taking your time should help with the precision of all those jumps, even if you take 10-second stalls between them. Anything matters here!
6-3X (From Beyond): Much like last time, exit to the map if you fall into the pit. For the buzzsaw boxes with keys in them, start with the one farthest to the left first, since it's relatively easy to die on that one. Then proceed as if you were doing the light world version of the level, doing the right key-box, then the top key-box, after which you should proceed to the right to your girl.
6-4X (Maze Of Ith): For me, the toughest part of this level is at the very end, with the saws moving up and down a series of shafts. Take as much time as you need, I reccomend not jumping until both of the saws in each shaft are lined up together. And be sure to let go of the run button for the middle shaft! Everything else should be simple enough.
6-5X (No Quarter): Don't play this one like an absolute madman. Take it reeeeeeeally slow on those jumps, as usual. The two toughest jumps for me are the jump through the buzzsaws on the middle island, and the jump immediately after that with the key at the top of a buzzsaw-lined shaft. The former jump is just practice, practice, practice (Remember, there's a safe spot you can use to stall on in between the two saws on the right side), but for the latter jump, and I know this might sound pretty stupid, do not actually jump to get down the shaft!
Instead, just hold the run button and fall down the shaft running, making slight movements to avoid the saws on each side. Everything else is just practice.
With these tipcs, I'm condifent that I can get this easily. My projected order, in case you were wondering, is 5, 4, 2, 3, and 1.