a risen sentry  

These undead frogloks roam back and forth in the two hallways between the Entry Hall and Mezzanine.

There are only two, one in each hall. To get from Entry Hall to Mezzanine (where the exit is) you can wait for the first one to emerge and stop near the front door, kill him, then invis to the walkway outside the second hall, wait for #2 to emerge, kill him, re-invis and run to the exit.