Xignal wrote in an older message (
http://www.tgeweb.com/ironworks/cgi-...ML/003330.html):
Playing through for the nth time I was stuck again in the trap door/elevator. Tried something different this time - carried a crate to the room, had my dwarf stand on it, and then pull the lever. Swoosh - slid to the Naga - no problem.
You might try this. Others have gotten by with enough wriggling, selecting a small character (like a pixie), and jumping up immediately after pulling the lever.
On the other hand, Wyv will get you through too.