Run away and wait for it to land, then move in and attack. Then have the tank run out to trigger the attack. I suggest that you have the ranged fighters fall back and hold their position. Note that it will land right on top of you if it can. The high dragon will come down and attack you. I believe that console players need to use the gong on the mountain top. Then right-click it and “Use” it to play the horn. You also need to have killed Kolgrim obviously.
On the PC version, you have to go into your inventory and find Kolgrim’s Horn. The bonus is that if you kill it you’ll have a dragon scale for Wade to make into really good armor. You actually need to draw it into a fight, since it isn’t in a place where it will attack you. Did you see the big dragon in that video? Well, you don’t have to fight it.
