Taxing 6, assigning skills to only 1 lemming, saving 90%
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The main trick is how to prevent any lost of lemmings for the leftmost entrance.
To do that, have the first lemming out that entrance walk for just 1 pixel, then
have him bash, but assigning him builder in the middle of his first bash stroke.
Time it right, and you get the result seen in screenshots A and B:  the partial
bash stroke takes out the top 2 pixels of the "step" at the base of the column,
allowing the next 2 lemmings out the entrance to get up that step.  This adds
enough delay for those lemmings for the digger's pit to become deep enough to
trap those two lemmings.  Make the digger bash after he has dug down 8 pixels.

The rest of the solution is just a one-worker variation of the standard 
"compression" solution, the 5 lost lemmings being one for each of the 4 traps
plus the blocker/bomber.  I included enough screenshots to show that when your
worker finishes bashing to the right, he's not positioned too close to other
lemmings from the crowd, so it's not too difficult to select him with the mouse 
again.  The screenshots also show that when your worker bashes to the left, no
one from the crowd will "leak out" getting ahead of the basher--just one lemming 
will overtake the basher, but he's able to turn around just in time before the 
final bash stroke occurs.