Hat jemand google angeschmissen und das hier gefunden. Ist leider in Englisch, aber hoffe, es nützt dir was:

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Level 23 From The Boundary Line
80 Lemmings (60% saved)
RR 99 5 minutes
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+ Bash through the wall using the first Lemming that gets to it.
+ While he's bashing, make him a climber.
+ When the climber is done bashing, you must have at least 49 Lemmings left
over. If you don't, you can't win, because 48 is 60% of 80, and that's what
you need to beat the level. If you have 49 when the climber is finished
bashing, go ahead. If you don't, start over. I say 49 because with the way
you're supposed to beat this level, the climber will have to die. If he's
the 48th one and he dies, you're down to 47, and that won't cut the
mustard.
+ When the climber scales the wall and is headed for the exit, dig a small
bit before the exit.
+ Build to make him turn around. Make sure that when you build, the other
non-climber Lemmings will be able to walk up to the exit.
+ Mine to the left so that the other Lemmings can walk up through the
diagonal tunnel and watch as at least 48 of them head for the exit.
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hier noch die Url: http://faqs.ign.com/articles/369/369161p1.html