Try activating some AoE spells before you enter, preferably the Storm of the century one or whatever it is. And when you actually fighting the Shadow things, go after the Weeakest party member first and leave the strongest till last.
So your best bet would be to attack your Mage first (if you have one) and then Tank last.
I find this fight is a lot easier if you have a Backstabbing Rogue with you, as He'll hit the most and do a lot of damage

mages need to use freeze. While they are frozen attack them with your warriors and rouges. On Nightmare this is a hard part. The only way I was able to do this part was to use freeze and than attack them.