Killed the dragon with a Ranger who had fast running shoes and a mage. Made a firewall with the mage and the ranger was kiting the dragon up and down the wall while the rest of the group killed the minions first and then stood in a save corner for the rest of the "fight".
As far as I could see the dragon only does his massive attack if he stands still, so keeping it in motion is the only option to not get killed instandly.