In the Zhent hideout, near the back where the two metal grate doors and the button to turn on the mines are - as you are looking at that wall, follow it down to the left. You'll get to a area where the path turns and starts heading toward a bridge. At that turn, hug the wall, and you'll do a perception check to notice the wall is an illusion. Behind the illusion is the elevator. It drops you right onto the Zhent storage area in the Underdark behind the Minotaurs. You can then get in some shots on them from above, making that encounter much easier. You can certainly find a YouTube video showing you exactly where this is if my description sucks.
