I gave up on pinnacle after like 10 tries doing different stradegies. Now I am fighting Mythal, and it is happening again.
I tell my people to focus on a weak leg, while I run around trying not to die during phase 1 and 2, sneaking in the occasional hit after a dodge. Once the dragon gets to about %70, they go to phase 3, fly up, land on a nearby wall, and shoot at me 3 times in 6 seconds, waiting 4, then starting over. My ranged person is still attacking doing damage(white blinks on their health bar), while I can sneak out and attack twice, before running back into cover. In the pinnacle fight, the cover is right in front of and to the left or right of the barrier, there is stone protrusions framing the barrier you can hide under/behind. On the Mythal fight, there are towers either side of the wall, depending on what wall, just on the other side of the tower, if you press your body up against the growth, she cant hit you... Anyway, I can dps the dragon down until I stop seeing the white blips on their health bar from taking damage, I assume there is a limit to how much damage you can do in this phase. My person is still attacking, but also not seeing white blips on their health bar.
The longest I waited in this phase is 20 minutes, it never ends to my knowledge. Then i just run out and die, just to make sure there isnt an on-death mechanic, and load the latest save.