Forest / Hospital / Factory / Hell / Rapture / The End / Cotton Alley
Light World:

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Dark World:

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Those would be mine, I'm still working on missile boy, I've got the light world 1-6 completely locked down.
The most obvious piece of information - start with the hardest levels first. For maggot boy I did 15-20 first, if you can do those without dying you can get maggot boy.
For brimstone boy I started with 18, 19, and 20. Everything else was static for me. Practice, Practice, Practice.
For anyone having trouble with Blood Cot boy, DO GHOST KEY FIRST.