Go to Murmurwoods and wait until June 24. Between 5 a.m.
and 9 a.m. look for the Unicorn King. Take the key from
him and return to Ravenshore. Then, find the fenced in area.
Note: That you must have gone and collected all the Obelisk
information first before going to Murmur Woods or the Unicorn King will not be there.
Immediately pay to have the Bow skill taught to all party members who can learn it. Next, buy them all bows; large volleys of arrows cut your opponents to ribbons.
Keep backing up away from your enemies as you fire off spells and arrows. This will allow you to get many shots off before they close the gap and enter melee.
One trick for beating overwhelming odds in dungeons is to hit the Combat button before entering the dungeon. This way, in Combat mode, you'll enter the dungeon with the action paused. Take a look around, kill a few enemies, and turn and exit the dungeon when you're hurt or out of manna.