Am I doing something wrong ?

I am at new game +4 and just entered dlc blind , my level is 232 and I am at skadu level 5 . I defeated the lion boss then defeated relana and then while exploring I drained water from a castle and met a boss named commander gaius. I am trying to defeat him for past 4 hours and whenever I reduce his health to 30% and he goes to 3rd phase I am not able to take one hit, he one shots me every time I don't know how much more vigor do I need my vigor is at 60 Or I should try some other areas first ?