OK, don't worry about doing damage to the boss in this fight that's not how you win. Throw away any minions/turrets (anything with a health meter) because the boss will heal itself by attacking those targets when it pulls everything in. The more targets you supply to the boss, the more it can heal itself. Stand off to the side on that little 'peninsula' next to the main circular platform to avoid AOE's. When you see the small white circle appear on the ground, the dragon should run over to it and stand in the middle of it. At that point, another larger white circle appears on the opposite side of the boss. Run over to that and stand on it for a few seconds and you should see your coordinated strike ability become available. Use the ability and then return to the point on the 'peninsula'. Lather, rinse, repeat.