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The three skulls are at

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Notice the North force arrows around? They are here for a reason.

×It's important to remember that you have 30 turns before the first spawn.
So, the overall approach is:
-destroy the southernmost blob on one side (as that's the one that will grow into a 2-by-n if unchecked)
-destroy all the blobs on the other side
-destroy the northern blob
-clean-up (be sure to keep the black doors in mind while doing this)

This means you need two skulls on one side, and one on the opposite side of the northern blob. The north blob is the hardest to set up; you basically need to lead a skull into one of the side passages, get it stuck there, and then lead it out from the other side.

I started on the west side and then went to the east. I think this might be necessary because of the black door layout, but I don't remember exactly.

×One of the skulls must be at “21,8”, but the two remaining skulls can be placed at “13,6”, “13,13” or “13,20”, in any combination. The configuration I've always used is: “21,8”, “13,6” and “13,13”.

×The first thing to check when entering the “5th floor” is that none of the skulls are stuck anywhere, either in the walls or behind the wrong force arrows. Then manipulate one skull at a time to “13,13”, “21,8” or “13,6”(and probably in that order), with the help of tar and tar babies, at the same time making sure that none of the other skulls are stuck where they are not supposed to.

When you've gotten a skull to where you want it, exit the room through the stairs leading up to the “6th floor” and then reenter the “5th floor”, as this resets the tar without allowing the skulls to move, which makes the whole thing a lot easier. Destroy the tar you don't need and repeat the process.