Found one of these at loc 236, 538. That's the south one of two chambers left and right off the main path through this dungeon, almost all the way at the end of it. Unfortunately 5 trash mobs spawned in the same room with the named.
The fly popped in the middle of the map for us, in the room at the bottom of the pipe. We were able to track it from the Zone entrance after we killed 20 or so. It didnt seem to hit much harder than any of the other monsters there. Dropped the rusty spring and a Putrescent Chain Bracer