Ah, useful glitch. A0;I can do Mayhem 12 now with 6 builders.
Actually, now down to just 5 builders on Mayhem 12. This should be the absolute minimum, barring some new unknown glitch.
This is slightly less straightforward then you might expect, especially towards the end. What you think could work is hampered by the fact (well, I don't know for sure, but it looks like it) that the exit's trigger area is only 4 pixels wide, so you need to find a way to turn a lemming around without using builders.
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As a balance to all these talks of glitches which I don't reveal in many of my recent solutions, I will now present another useful fact, something that I'm actually finding myself do more and more with the recent challenges.
In CustLemm/Lemmings/ONML, the basher moves forward exactly 5 pixels for every bash stroke.
So for example:
O....X....W
If the Lemming is standing at the position marked "o" when you assigned him the basher, one stroke later he will end up at "x", and then at "w" after the second stroke. Each character above represents one pixel.
Certainly not earth-shattering, but I certainly have found myself doing a lot of sketchings lately based on this. It's a good way to find out where your basher will end up, or a good way to figure out where exactly you should start bashing in order to end up at a certain place you want.
Now, the basher does move continously. It does not skip the intervening pixels. However, the nice thing about it is, the basher is stationary while it is removing terrain, which takes several frames to complete. It's clear how when you look at the animation, there's a phase where the basher is standing still doing the stroke, and then followed by the phase where he moves forward in preparation for the next stroke.
So while in theory you can assign skills to the basher at its moving phase, it's obviously more preferable to assign him a skill at its non-moving phase, at which it'll stay put at one location. Which is why it helps to know how to calculate where you should start bashing. Then you only need to count how many strokes you need to wait for, and then at the start of the stroke after the count, assign the skill, and you're guaranteed to have the skill assigned at the location you want, provided you started bashing at the exact location you calculated.