I also had problems with this guy, especially the two archmages are really hard. After some trial and error, i removed them from the fight with an alpha strike: give everyone the slowest, heaviest, hardest hitting ranged weapon you can find in their secondary slot (the ones that take ages to reload). Immediately after the fight starts, hit pause, have 3 chars fire at archmage1, the other 3 at archmage2. If all goes well, this kills both archmages. Immediately switch all characters to their normal weapons and resume the fight normally (i had my spellcasters target the priest next, after that the worst is over). Still a hard fitght after that, but no longer impossible.