Have been trying to get Sundancer in Bastion using my Dracthyr and Druid. I can go to the Ancient Memorial and get the buff but once I am able to get on the mount the extra action button does NOT pop up. I could be wrong just going by what video shows. But it stated a action button would pop up keep pressing it until it take it to the Ancient memorial where you will have to fight it. Instead as soon as I land on it WITH SUNDANCER BUFFF ACTIVE it kicks me right off.
The only obvious thing I don’t see in your description is how you used the required Skystrider Glider to get over to mounting Sundancer. Because if you didn’t, the “Soothe” button not appearing is intentional.