There's two thing I did that let me clear the fight.
The first is the attack pattern. She starts with Behind Cover, but she loses it when she counters. So one of my first actors (a frontliner) would provoke a counterattack so the rest of the team could hammer on her. The other combatants aren't nearly as much of a threat, though if you wanted to you could blind her then mop up.
The second is gear. Between manor upgrades and locations, I had good access to steel and ironwood, and some access to magic wood and mithril. Pushing my team through a full gear upgrade closed the gap.
As a detail, bringing Zephyra is a huge help because her exclusive Wind Wall skill counters arrows pretty well.
For the record, I'm running a dominator/necromancer build, with an orc companion berserker/deathknight. I ended up training up a dragon knight to assist, and with those three plus Zephyra I cleared it pretty solidly.
Good luck!