Level 16: Don't do anything too hasty[edit]
Make the first lemming start building stairs to the exit. He will need several bridges to reach the exit. Place a blocker on each side of the entrance to keep the other lemmings from following. When the builder is done, remove the right blocker and the crowd will join the leader in heaven.
Level 17: Easy when you know how[edit]
This level looks simple at first glance, but there are four traps hidden in the long passageway at the bottom of the level. Avoid this area altogether by placing a blocker near the left entrance. The easiest solution is to assign all 20 of your floaters to random lemmings and then bash through the four pillars. The 20 floaters will be the only survivors, but they will be enough for you to meet the 40% save requirement.
If you want to save more lemmings from beyond the quota, you can place a blocker just after the first drop on the left hand side, and collect as many lemmings as possible just after the drop. When you detonate the blocker, there are more then enough lemmings in a tight group to pass through the traps, as these traps only kill one lemming at a time.
Alternate solution[edit]
Quickly place a blocker at the left side of the platform to make this section safe. Move to the far right side of the platform and begin bashing through the pillar. Place a blocker a little way behind the basher − you're aiming to separate off the basher plus one or two others. Make these all floaters before they fall.
When the first of them gets near the exit, make him a blocker to turn the other(s) around. Then set the other(s) to building a staircase from the exit "hill" to the side of the platform, making the drop survivable. When the stairs are complete, explode the blocker near the exit and the one on the right of the platform, and have the other groups bash through their pillars. This won't get you 100%, but you'll only lose the blockers.
Level 18: Let's block and blow[edit]
This level is divided into several compartments, several of which have fire inside. The goal here is to get to the bottom of the level without entering any of the compartments with fire in them. To do this, make a lemming block when he is directly on top of a compartment without fire in it, and then blow him up. Repeat this process until you get to the bottom of the level. If you make a mistake, you can also use blockers to keep the lemmings out of the fire.
Level 19: Take good care of my Lemmings[edit]
Make the second lemming block after walking a few steps. The third lemming will turn around; once he is left of the entrance, make him block. The first lemming will walk ahead. When he is about 2/3 of the way over the last hump, make him start building. 4-5 bridges will be required to reach the exit. When he is finished, remove the blocker on the right.
Alternate (100%) solution (warning, this is Tricky if not Taxing difficulty)[edit]
Wait until the lemmings have reached the last hump. Make the first build from just past the crest, then the second block at the crest.
When the first lemming to return to the left is about halfway up the right shoulder of the third hump (the last solid one from the right), make him mine until it is no longer possible to climb out of this tunnel to the left (the following lemmings shouldn't arrive soon enough to get past this tunnel before it is long enough; if one does, turn him back to the right by digging a shallow pit and then building), then turn him into a builder to stop his mining. (The problem is that this lemming tends to reach the third hump just as the builder is running out of bricks, so some nifty finger/mouse work is required.) When (or just before) the bridge to the exit is complete, undermine the blocker and then build to stop the mining (and possibly to be able to get out of the resulting small pit).
Level 20: We are now at LEMCON ONE[edit]
Have the first lemming build over the gap. The second lemming will catch him before he is finished; have him build, as well. Let a lemming walk down the stairs on the other side of the gap and bash through each of the seven poles. Finally, build over the final gap to reach the exit. Use multiple builders to keep lemmings from falling in before the builder is finished. Don't worry if you lose a couple, though.
100% solution[edit]
As above, have the first two lemmings build over the gap. Then, as soon as possible, on the other side build an impenetrable wall from three (or more) ramps packed tightly together. Make one lemming an athlete and have him climb over the first pole, then bash through the rest. Finally, when the athlete is building over the last gap, bash through the first pole.