I beat it with a "level 10" party but unfortunately it's because my game is super bugged.
I suspect my tank is about twice as tanky as he's supposed to be because of that stat stacking save game bug - god I hope they fix that, I'm way too far in to want to start again and it's going to make the rest of the game trivial if he remains like that.
And I also have 3 Wurms that my chanter summoned about 20 hours ago that I have no control over and just randomly appear somewhere on every map I travel to. They're still friendly to me, still draw aggro and are also basically immortal as they just get back up again every time someone knocks them out. On this particular map they spawned in on top of the Adragans so I never had to deal with them the whole fight.
So on the whole, I don't think it really counts...