I can't remember my exact tactics, I'm sure others will pitch in with more detail, but I recall a couple of things that helped. Firstly -AP curses, get them down to max of 1 attack per round, this gives you a chance to heal your party. Secondly, run away! Try to only deal with one or two of them at a time so you can focus your attacks. If I remember correctly, there are also walls you can back up against, reducing the angles you can be attacked from. I think I also made a point of taking out the ranged attackers 1st.
If vegetarians eat vegetables... What do humanitarians eat?
This was indeed one of the hardest fights. In my case I did more or less what the others described above: cursed them to cut down the number of attacks and ran off to the northeast corner where there's a ruined wall with a narrow space between it and the main wall around the area. I put Kjartan and Tamilin in the middle, so that they couldn't be hit except by ranged attacks, had Kyera block the furthest end and Vraes the closest one to bear the brunt of the attacks. That way they were forced to split up and couldn't attack one hero all at once. Also, make sure to attack the ranged guys first because they're far easier to kill, focus your attacks on a single opponent as much as possible so that they don't have many opportunities to attack you before they get killed, invest in curses and buffs (they really make a difference) and tank up Kyera so that you can hold your own in a two-pronged attack.