Make the first lemming a turner to the right. The lemmings will go through the teleporter.
There you build stairs over the two holes and your lemmings get teleported again.
Bash through the one-way block and have the lemmings turn to the right.
Build stairs over the teleporter and place a blocker at the end so that the lemmings turn and walk through the teleporter. Now they slide savely into the exit.