I replayed Big 20 #9 games recently and remembered these notes I had made, but never posted. I'd suggest saving the marked map images in the imgur link below because you never know when they'll disappear.
Stage 1: If you get a skeleton in door 1, the bomb is in door 4
Stage 2: If you get a kid in door 2, the bomb is in door 3. If no bomb in door 1 or 2 and no kid in door 2, go to door 4.
Stage 3: Make sure to let the 3 green guys in the jungle follow you -- they despawn several enemies until the bird section
Stage 4: If there is a kid in door 2, bomb is in door 3
Stage 5: If you get skeleton in door 1, it is Door #4 If you get a kid in door 3, it is door #7
Stage 6: If skeleton is in Door 2, it is Door #3 (this is bad)
Stage 7: No one knows how to manip the mashers. :( If kid is in door #1, bomb is in Door#3
Stage 8: No hostage.
Boss: Jump towards boss at the start of the second chair, crouch, then mash shoot.
