From my one month experience with Lost Saga, I find that all the characters are pretty balance in a sense that each of them have their own usefulness. The reason why you people find X class OP is due to the lack of match up experience I would say.
With regards to zerker, he isn't OP at all. The thing you should take note of when facing a zerker is to predict his horn skill and use a skill which is longer than the horn duration or to simply jump away from his horn aoe range. That is provided you didn't get knockdown from his or his allies attacks (If you get knockdown be prepared to eat one full combo.. )
Alright moving on to boxer. I agree that his attack speed is gay and he can edge you easily with just punches. However, why in the first place would you go 1 on 1 with a boxer? If your skills are cooling down just jump away in between blocked attacks. Save your skills and use it on reaction when he uses his champion belt and his nuclear punch or just use your skills to break his punches.
I am a viking user by the way and I don't find a particular class being OP. You just got to learn the different matchup strategies against the different characters. Good luck.