Pinnacle of Its Kind not completing

Thanks, I'll give that a shot when I get home. The two quests have some overlap and it's a bit confusing - but I'm quite sure I've already fought the High Dragon boss version of the Formless One in addition to the Revenant Dragon in the Crossroads...I'm way too far into the game to reload all the way back before to check that though. But I'll try interacting with the brazier and report back.

I wish there was a better way to keep track of what you already did - unless I'm missing something, the local maps don't seem to have any names on the areas or the beacons so I'm having a hard time recalling what I've already discovered and killed versus what I only investigated but could not yet complete and I keep having to retread old ground to find out.

I also have a problem with this quest. I killed all 3 demons (yes, the quest is now updated to go back to the sealed door) but the door is closed. When I approach it, Rook doesn't say the wisps are needed to unlock it, nothing. There are 4 wisps clustered floating around chests next to the merchant, but not sure whether they are there by design or are they the ones for the door misplaced somehow. They don't follow me at all

I'm having the exact same issue. I even thought it was immune to lightning damage, so I switched up my build, only to get the sxact same result. The quest is definitely bugged. 

EXACT same problem. I've tried many times, he goes up on the perch and doesn't move and the health stops just like yours. Someone needs to fix this. I got so tired of trying over and over again that I got a trainer and put on super damage just to see if I could kill it before it perched, but nope. He just goes straight up there and the same thing happens, no damage. So there is literally no way to kill him right now. -.-

Same issue here...

I have killed only one of the demons and the second one in Dock Town after removing his barrier and armor... He keeps on healing to 100%

I can barely get him down to 95% and he just heals to full out of nowhere... I even thought it was a display bug, so I kept fighting him for like 30 mins (because I play on Nightmare) just to see if he will even die... Well, he did not 😂

Same problem here

As a workaround I don't go to Necropolis after recruiting Emmerich until I have killed all Champions in the world as xPapiCereals69x suggested in comment below. Then I just do all demons and the final dragon in one go to make sure I don't  need to go through the door anymore. This way there are no quests overlapping in some areas and the door in Necropolis actually stays open.

I'm having the same issue too. It's crazy to me that EA hasn't fixed this yet. This seems like something that should have been caught in beta.