And this is how I improved minimac's challenge:
First lemming blocks, second builds off to the left, third is a pass-through blocker. Builder falls to the left-hand L, builds, bashes to release blockers and crowd (importantly, by now all 20 lemmings are out, so the worker lemming has time to build over the gap while everyone else is going to the left). As soon as the worker crosses the gap, bash to stop him going too high.