Psenemphaia Hideout
A bandit encampment east of Lake Mareotis, in the Sapi-Res Nome area.
A bit of pushover, this one- the treasure is just out in the open by the crossroads in the middle of camp, and there's only a single guard posted at night.
Start by sniping the guard.
You can then kill the captain in his tent. If he's too high level for you to just cut his throat (and how does that even work?), at least a stealth attack should make a giant dent in his health- easy enough for you to finish the job normally (though you'll probably want to sleep-kill any nearby bandits first, to avoid a commotion).
Snag the loot in the middle of camp for location completion.