I would:
1. Wait until Crusher is standing on the bridge outside
Push him down and pocket the ring. The other goblins are only exp bags (and the ring can't be stolen)
2. Use various opportunities to kill goblins in "blind spots" around the fortress. This should rid you of at least 10 combatants
3. If you're far away from the Zhents when starting combat proper, and long rest after point 2, they'll run away
4. Get Halsin from the cellar
5. The only enemies left are Ragzlin, Minthara and some minor goblin bosses (who can all be taken out using stealth and timing, but that's tricky).
6. Get Halsin
7. You now only need to take out a lone Minthara and Ragzlin
This approach is over cautious, but it works if you're under leveled.
