This quest is still available after getting the battle tower (as is the one on sentinel island to retrieve a book from a ship in a tree, though you may want to consider reading the book yourself rather than completing the quest).
I entered the quarry building at level 21 or 22 and with my creature as a distraction was able to take out the 2 downstairs (for ~2.5k experience each). After going upstairs I could not handle the 4 there, so had to reload. At level 23 I was able to rescue Williams and take out the 4 rangers that come upstairs (~9.7k experience total), but not the 3 warriors downstairs, so I had to high tail it out a side door. That was the last place I could clear before getting the battle tower; by the time I got back the 3 guys downstairs were trivial to defeat.
