I am a new member here. But I just need to bump this topic because this could be a helpful post =P
Here is a very simple way to beat Final Impediment. I've made a screenshot and uploaded it to my site(very bad site, and it goes about another old game. It's called Jazz Jackrabbit 2) but you will find a screenshot with how I did Final Impediment. I could do it simplier, but this is good enough
Here a short explanation :
First Lemming starts building from the right side of the platform, till he builds against the wall(he won't get hit by the fire) when he walks down you should start building to the left when he is at the 12th brick he built. Then, after doing that, try to mine the blocker free but if that is unsafe, try to build over the blocker. Then let the last Lemming build to the left so he walks to the right. Let him mine the blocker free. You need to time well! And you have to change the miner into a builder. If done correctly, you will have the same as my screenshot, found here :
http://www.freewebs.com/darksonicjj2/morescreenshots.htmAs for I am A.T. : I've completed it once but I had alot of bad luck already. It's hard to get that 80%. I have had 77 and 78 percent, but I completed it once with 80%.
Hopefully this post is helpful.