Use a Floater skill on the very first Lemming, and then, a Climber skill. When he is up on the top of that silver bar with the sphere, use a Builder skill to turn him around, then another one to that platform above the trapdoor. Meanwhile many of your Lemmings will be dropped out of the trapdoor and splatter - you can't do anything about this yet. Wait for your Lemming to reach the center of that cube-shaped object near the trapdoor, then start building the ramp. You must build the ramp long enough to reach that silver bar your Lemming climbed earlier. You must preserve 1 Builder skill for the end of the level, however. If you couldn't, you may kiss this level goodbye now.
When the ramp is ready, your Lemmings will no longer fall down and die. Let'em walk back and forth on that ramp for now, wait for the Climber-Floater guy to get to the top of the silver bar with the sphere. He can walk forwards without problems. When he reaches that crystal, use a Basher skill. Allow him to scale the chainlink structure to the exit, but watch out for that bright-coloured thing on the floor - it is an antimatter beam trap! Any Lemming that steps on it will be blasted to subatomic particles in nanoseconds! So you have to use your last Builder skill to create a staircase above this trap. Once your Lemming successfully reached the exit, use a Basher skill on one of the Lemmings in the crowd to smash thru the silver bar with the sphere, and start marching towards the end of the level.