Zog was Level 37 and I was 29 for this fight. My melee attacks did not work. Looking back I could have just stealth assaulted him again and again by leaving and coming back. Zog took 11 minutes to kill with this attempt, so it was very tedious, relying on arrows until the end where I got in a stealth attack. Initially I forgot how to crouch and messed one attempt up.
Nothing exciting but an effective method if you are having trouble. Though I would reccomend stealth attacking, retreating and repeating. I think that would be much faster.
I believe I died once on Zog. For the pillars I had to purify, I tried to just run up to them and hold B immediately when I was able to and minimise fighting. The usual strategy I use for those segments.