I killed Zagan last night. First destroyed both nests (or 1 nest two times... dunno... too much flashing), using dragon heal skill in meanwhile. Then took human form and rested in upper tunnel. There were only 2 wywerns and Zagan left, so i was just flying around and spamming spheres until all were dead.
I used step-by-step saving method. First nest down and enough HP - save! Second nest down, save again under different name if u like and so on. Then u dont have to start it all over again every time.
Tried also to target Zagan right away but after killing him all that i could see were lots of red dots on minimap (wywerns) and flashing lights around me and then i died.

IMO game is unbalanced a bit - all ground melee-fights (incl bosses) are even too easy and dragon fights are way too hard. Or is it only me?