Crazy 18: It takes two builders (and some perfect bridge-stretching) to get from the upper platform to the top ball, from which the lemmings can walk to the exit. As far as I'm aware, there's no viable 1- or no- builder solution using the Tame 20 trick, since the time limit and skill set are too restrictive. Every other skill, however, can be eliminated by varying the 2-builder solution in different ways.
Result: 2 builders.
Crazy 19: One builder is needed to get across the water, and other routes don't offer any improvement on this. The other skills can be picked off one by one without much difficulty. It's helpful to know that you can build over the one-way wall with three builders.
Result: 1 builder.
Crazy 20: 50% requirement means you can ignore the left side altogether, which helps a lot. Blockers and bombers are easy to eliminate. 6 builders seems to be a minimum - unfortunately it takes two to cross the first gap because of the spike, and there's no way to get rid of in such a way that keeps the builder moving without losing a lemming, and thus needing to save some from the left. Bashers can be left altogether by using bombers instead, and then saving the left side using the spare builders.
Result: 6 builders.