Hints:

Pause the game to give a skill to a lemming, particularly if he's near a crowd. You can tell which lemming will get the skill because he'll be highlighted when you hold the cursor over him. The game will immediately unpause once you click him.

Use blockers to corral most of your lemmings when you have to watch a particular activity closely, such as building stairs or timing Bombers.

Some levels appear impossible until you figure out how the triggers work. Do a test-run over all the laser and levers before you give up