"I am stronger than you."
Translation:Jestem silniejszy od ciebie.
Well, firstly: "silniejsze" is a neuter singular form. Generally most people are either male (silniejszy) or female (silniejsza). I can possibly imagine a non-binary person saying "silniejsze", but I do not believe that there's just one way that all Polish non-binary people refer to themselves and using neuter to me personally seems quite offensive. But well, what matters is whatever the given person chooses, if they want people to refer to them using neuter gender, then that's their decision.
Secondly, "wasz" is a form of "your", not of "you". It should be either "ty" (singular 'you': than you, John) or "wy" (plural 'you': than you, John and Susan).