Taxing 4, assigning skills to only 1 lemming, saving 100%
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There are 3 notable parts in the solution:

1) Bypassing the rope trap:  if you dig at the last safe pixel left of the trap's
trigger area, the digger pit will be deep enough fast enough, and extend far enough 
to the right, for the next incoming lemming to avoid the rope trap.

2) Keeping the crowd from following your worker while he's building at the area
with the spike traps:  this is important for saving 100%, because otherwise the 
crowd will inevitably be harmed by the spike traps when they fall off a still-
forming bridge close enough to the traps.  In my solution, I isolate the crowd
from the worker by having the worker dig down a little after he builds to the
wall and have just turned around.  This will break the bridge's connection to
the wall, so the crowd will fall off the bridge instead of continuing up to
the next bridge.  The worker then simply resumes building.

3) Getting back to the lower part of the level to release the crowd:  you do this
by mining to the right just a little left of the exit trigger.  This will bring
your worker back down to the area right of the rope trap and nearby steel wall.
Then mine and bash to the left to get back to the area where the crowd is.  Since
there are only 40 lemmings spread over a relatively large area, it should be
pretty easy to make it so that your worker lemming is nowhere close to any
lemmings in the crowd.  Your worker can then be easily assigned builders to
repair the bridge breakage from part 2, and then to reach the exit trigger.

I try to include enough screenshots to demonstrate that at all the moments when
the worker isn't fully isolated from the crowd, you can nevertheless select
the worker with the mouse without too much difficulty.