What you need to do is take control of your tank at the beginning of the match. ALWAYS start off with your tank. Run into the crowd and use taunt to aggro the mobs to you (make sure you go for this skill as soon possible). Then select the strongest enemy and have your tank attack it. From there, switch off to your dps and have them attack whatever enemy is NOT being targeted by the tank (tank will hold the mobs attention due to the aggro move).
You also want to make sure you have lots of health potions, even if you already have a healer(have one of your characters go for the herbalist skill as soon as possible). Make sure you get at least 2 different tiers of potions(lesser/regular, regular/greater, etc), since you can use both at the same time(they don't share the same cooldown timer). I personally have 3-4 different tiers of potions, but that's just me.
As for AI tactics, have all of them (except the healers) start off with:
Self: Health < 50% Use health poultice: least powerful
This should give you enough time to switch characters and pop a stronger potion if the characters are getting pummeled.
And finally, throughout the fight, make sure you constantly go to your tank in order for him/her to pop taunt. You can create a tactic for it, but I find it easier to manually activate the skill.