I tried fighting Zarathax 3 times today, and all of the tries failed. First of all, why does his minion get to use storm shark? If he is a myth type, he should use myth spells. If you want to fight him as a level 40, you would have to have a full group of four. And Zarathax shouldn't have judgement. He had one pip and no blades or traps on me, and did 3 hundred damage. If it were a regular player, it would only do 1 hundred. Kingslse, if you are reading this, please fix this. Goodbye
Try again, change your deck, optimize your gear, etc. Just beat Zarathax a few days back (solo) on my level 37 , just keep trying at it and if possible team up. Most of all I would recommend creating a game plan, he is certainly doable and you have many more bosses after him. Build your deck around what spells he uses, right now there is free respec, so make sure to use of that to get spells such as Tower Shield and maybe Feint (if you haven't) and play a bit more defensively. Good luck!
, just keep trying at it and if possible team up. Most of all I would recommend creating a game plan, he is certainly doable and you have many more bosses after him. Build your deck around what spells he uses, right now there is free respec, so make sure to use of that to get spells such as Tower Shield 
and maybe Feint