I managed to clear out the pits at my current fortress for the first time ever.
It was a pit of tentacle demons, and I knew I was getting close as the adamantine veins were getting closer and thicker. So I built an "encounter room" with fortifications and got 5 of my champion legendary hammer/marksdwarves ready to accept the demonic charge.
On a related note, I had dug a zig-zag channel in front of the fortifications so the demons would take longer to get to the wardwarves.
When I breached the pit, imagine my surprise when a blood tide of crazy peasants came flying out! However, my zig-zag channel served as a pit-trap for all those crazy people, and so they all ended up in a tiny little area. The tentacle demons though were invisible until they reached my first Dwarf, where a wall of steel arrows began an epic and terrible slaughter among the tentacle demons.
Final score? 35 dead tentacle demons, not a single injury among the wardwarves. And fun extra names for the dwarves.
After a while, the crazy people died, and I had a vast and epic number of bones to train more marksdwarves. And demon skulls make valuable totems to trade.
When it was all said and done, the pits were clear, I was able to mine every last bit of adamantine, and my FPS went back up to almost normal!
A note, how do I get rid of all that (filth) and (unknown substance). It appears to be a liquid, but nobody will dump it. I have buckets, but no go.