you can find tips for doing so on this forum:http://social.bioware.com/forum/1/topic/308/index/6645696/1
Have all of your squishies hide behind the pillar with crates, then as soon as the red beam is over, have your tank run out and focus fire them down. Have your party on hold so they're always behind the pillar, so your tank is the only one out in the middle.
You can also do it the quicker way, which is nuke the boss when he's in his dormant form, then when he gets up, pick off adds with Varric's auto attack.