Have you actually tried that? Cause it doesn't work for me. My main character always ends up in front, no matter who I place in front in the custom formations, and no matter how I rearrange the character icons. Pretty bad, if you play an unarmoured rogue archer ...
Made a ]thread about it in the support section.
As for tactics:
I beat it by putting some plate armour on my rogue, which let her survive the fight despite being forced into a disastrous position by the game, and equipped each and every character with a slow-firing ranged weapon (arquebus, pistol, blunderbuss, arbalest). Let everyone attack wyrmund at the start of the fight, which will take off most of his HP (and a lucky arbalest shot might knock him down). Then you can swap to melee weapons with your tanks, and finish off wyrmund with spells/secondary ranged weapons with the rest of the group. Once he's down, the rest of the mobs shouldn't be too hard to kill.
I'm playing on hard, iirc. Might not work as well on the highest difficulty...