*Spoilers Ahoy!*
The attackers from the immediate north of your position include thieves that will stealth and try to backstab your back lines. So a Detect Invisibility wouldn't go astray. And have any of your fragile spellcasters cast Mirror Image if possible.
Then off to the left are the Really Annoying Archers. I believe it's their arrows that poison (like Hobgoblin arrows). One solution is to get a front line fither close enough to them that they switch to a melee weapon (and they have lighter armour, so they should go down pretty easily after that.)
Then there are the Whimpy Sounding But Actually Pretty Tough Knights. The WSBAPTK's spawn in front of the RAA's, and I think maybe a pair lower down from your party's entry position. I find that magic works well against them. A Greater Malaison plus Horror or Emotion or Chaos can turn their well oiled attack machine into a duck shoot.
Is there a spellcaster in that fight? I think he just runs away. But if there is one, have your best archer concentrate fire, and have one of your mages Magic Missle away his/her mirror image. But I don't remember that fight having a mage.
Oh, and don't enter the fight exhausted. Rest up maybe at the zombie farm, select your spells, save, and enter The Beard fully healed and unfatigued. If this feels like metagaming, remember that silly Dwarf going around shooting his mouth off about his grandma's soultaker dagger - the high Wisdom characters in your party would suspect that his tales may have inspired unfavorable attention anyway.
Oh, have ALL of your characters have an antidote or two in their quickslots. Go buy some from a temple in Naskel or Beregost before returning if you don't have enough. Poison kills comically fast in this game, and it'll save you some trouble.
One last tip: Haste. AS SOON AS the screen loads, pause the game and have your mage start casting. I think you can get in the first bit of the spell before the dialogue.
Okay one more: Charm related spells. Dire Charming a couple of the WSBAPTK's can make the fight a little easier.
Aw, heck. When the enemies finally appear; pause and study their position. Who is where and how many. You would have been able to see most of these guys coming from a few miles away anyway (a la the Sir Lancelot charge in Monty Python and the Holy Grail). ;D