As charming as it may seem, do NOT let your Lemmings wander to the right - they'll fall and splatter. Instead, set up a Blocker at a spitting distance from the trapdoor, and guide'em to the left, towards the ribcage. When they all fall down, order the last Lemming to mine under the Blocker and set both free. Give them Floater ability, and allow them to fall either left or right. You shall need only one Lemming to reach the region of the bridge (alternatively, you can let'em all fall down to the left and blow up the Blocker).
Utilize a Basher to smash his way through the soil towards the bridge. Use another Blocker here, allow only 1 Lemming to pass, he shall have to build the ramp towards the exit. The ramp can start right were the bridge is, however it's not a problem if you start building the ramp near the second log. This'll take a few minutes, but is possible. When the ramp is finished, either blow up the Blocker or mine underneath and set him free, so you could score 100%.