Okay, if you don't want to know the whole solution, I'll just give you hints. At the bottom of the post will be the solutions (the ones I used, at least).
Mayhem 19 - Get Down Tonight
You don't need to contain the crowd.
Mayhem 24 - Back and Forth
You need to go under the one-way wall.
Both levels can be done in under one minute, and are both much simpler than they look.
Okay, now the SOLUTIONS:
Mayhem 19 - Get Down Tonight
Bash through the wall to the right. Make the lead lemming float at the latest possible moment and build as soon as he lands. Also make the lemming behind him float. Then make the first lemming to pass the builder block. The bridge will catch the rest of the lemmings and the blocker will turn the builder. If you want to get them in in less that one minute, take the release rate to 99.
Mayhem 24 - Back and Forth
Make lem2 bash to delay him, and lem1 build over the gap. Make one of the last lemmings to fall from the bridge before it hits the wall dig in the corner and bash when he's under the one-way wall. If it's done right, he should be alone or have one partner. Make him/them build over to the exit. You will probably need two builders even if he's alone. Finally dig to release the others.
Hope that helps!
