Rabid Jotaz?

[Mister Shan]

Is this guy optional? Is he someone you come back later to kill? He is absolutely impossible. A really drastic difficulty spike and really cheap. The guy one shots me.

[3dRes ｼ]

He's an optional miniboss, and you can come back to him later or after finishing the game. Sounds like you just met him, best to come back when levelled if you are having trouble

[Olga 1987]

It's been really difficult until I understood how to fight against him: precisely done dodges are the most important thing (e.g. he likes attacking two times in a row, so the dodge must've been done twice), but it's killable even on maximum difficulty. But yes, he's optional; you have to kill him only if you want a certain Origin achievement for killing all Legendary creatures.