Let the first Lemming walk to the island's edge and apply him as a Blocker. Apply a second Lemming as a Miner, towards the right of course. Use two Climbers at the one-way rock, and of course, another Blocker near the void to the left. Using a Basher under the one-way rock is a dumb idea, as the ground will lead to a bottomless pit, from which there is no way out.
Watch the two Climbers. When they reach the area of the exit, apply the first as a Blocker, and the second one as a Basher to destroy the one-way rock (use a Miner skill near the left side, so the Lemmings could advance). Now all you shall need are a couple of Builders near the rightmost Blocker to make a staircase leading to the exit. Even without saving the Blockers, you can easily score 93%.