Before hunting the next dragon I made sure to pick up the final surviving follower from Death Mountain: Zuna. She was terrified to come with me knowing the fate that had befallen all the others that did, but she agreed provided I'd help her look for her brother Xuru at some point. I agreed and so, we confronted Ziisizaan the stamina eater. Because he'd literally sap all my stamina away just by flying over head. This was only part of the problem though. The other problem was he was invincible. Yep! Nothing could hurt him!
It took a long while before finding out that his invincibility was some sort of shield. When he shimmered white, he was on god mode. The trick was getting him to land where I wanted when the shield was off. This is more challenging than it sounds since it's like he's invincible 90% of the time and prefers to be airborne. It also doesn't help that at half health he flies off out of sight and returns fully healed.
The silvery glow is actually his shield.
This made for a very long and annoying battle. Finally I just had to resort to convincing him to land without using "dragonrend" (by running around like a fool) so I could instead "Slow Time" and power strike him, drinking a gallon worth of stamina pots to actually deal enough damage to kill the scum before he flew off to regen to full health. Phew, obviously the enemies were learning to cheat too!