Start on the right side.  The first lemming builds to turn around and then builds at the water.  The second and third lemmings on this side bomb in the same spot to create a holding container for the crowd.

Meanwhile, the first lemming on the left side builds over the water, starting from the very edge.  One of the later lemmings builds to turn around, and then builds just before the drop on the left (ignore this far left lemming when he runs out of bricks).

Now focus on the two builders going over the water, one from each side.  Make each build again when he runs out of bricks, but make sure the left bridge is stretched as much as possible.  The right builder will hit the left bridge and turn around, while the left builder hits his head on a small vine and turns around.  When he gets to the bridge on the far left, make him build about halfway up; the goal here is to make a safe fall to the rock below.  Once he drops to the bottom of the level, just two more builders get him to the exit.

The only thing left is to release the crowd.  This is done with a single bomber, who explodes just to the left of the crowd's container.

1: Make the first lemming bash through the monster's head.
2: When the basher finishes, the lemmings will drop down to the very bottom of the level.  Make the first one dig as soon as he lands, and them block after 4-5 swipes.
3: The lemmings' forward progress is hindered by a spike.  Make a lemming mine left as he turns off of the spike; this will allow a climber to pass.
4: Assign a climber and make him bash shortly after he climbs the spike.
5: Assign a second climber and have him bomb in the place shown in Screenshot A.  He will remove the rest of the spike, allowing the crowd to pass.
6: After the basher finishes, he will climb over three spikes.  The fourth one will make him turn, so bash before he gets to this one.
7: Assign another climber, such that he turns around in the basher's tunnel.  Make him mine left toward the crowd.
8: When the basher finishes, make him build from the second spike.  He will hit a star and turn around.  When he returns, build from the top of the star.
9: The miner should be finishing up about now; but there is still one more spike preventing the crowd from continuing.  Make the lemming a digger at the top of this spike (Screenshot B), and make him block once the crowd is able to pass.  The blocker will have nothing to stand on, and will immediately transition to a walker.
10: The builder from Step #8 will run into another spike and turn around.  Shortly after he turns back to the right, make him a bomber.  He will go off on top of the spike, removing it (Screenshot C).
11: The crowd should be arriving right about now, but they will turn around on the final spike.  Make one of them mine left as he turns off the spike; his backswing will make the spike climbable.
12: When the last climber (from Step #9) arrives, he will climb the spike.  Make him a builder as soon as he reaches to top; if done properly, he will seal off the one-pixel gap he just climbed through.
13: Make this lemming a blocker in a pixel-precise location, shown in Screenshot D, and then bomb him.  His crater will take out the rest of the spike and create a ramp for the crowd to proceed to the exit.