Taxing 5, assigning skills to only 1 lemming, saving 98%
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The main trick is how to isolate your worker lemming from the crowd.  In my
solution, I accomplish that by having the worker dig, bash, and then build to
stop bashing and turn around.  Then when the worker has just turned around
against the left wall of the pit, have him build 5 or so steps, then dig down
2 steps, and then continue building.  The digging will break the bridge so
the crowd can't follow.  The building will get all the way up to the thin
ceiling of the basher's tunnel, allowing your worker to be up on the surface
again, isolated from the crowd below.  Now your worker can work in peace.

Your worker will construct the lower-route solution as seen in the screenshots.
Except when you get to the final set of bars just before the exit, the worker
will not bash it right away.  Instead, he will make a path back towards the
crowd and free the crowd via bashing.  Your worker will naturally be a little
distance behind the crowd when he's done.  Now just wait for the crowd to
turn around against the final set of bars, and then your worker approaches
the bars again, and now is the right time for him to finally bash through them.

I included enough screenshots to try to show that at all the moments when your 
worker hasn't been fully isolated from the crowd, you can nevertheless select 
him with the mouse without too much difficulty.