The Guardian of Leadership has a very serious arcane d.o.t. that will kill the group very fast if the d.o.t. is not removed. Make sure you have a person in each group that can remove from group with one shot, or enough mages to cover the dot duty removal.

Keep healers alive through this fight. Pull the dragon into corner and have raid group back away from corner just behind his tail, careful to avoid the door and any adds remaining.

He's a slow burn but will go down.