Best way to deal with 'that guy' is just say 'and how do you know what everyones dps is?' since parsers are a bannable offense that gets them to shut up 9 times out of 10, the other time they usually call me a 'casual faggut that needs to gb2 wow' -which is funny because usually they have a name like "arhtass lichking" or "darklady silvanus" and they drop group.
I actually watched an unintentionally fascinating video a while back where some WoW arena players were talking about FFXIV's pvp saying "i love it but im banned for calling others faggots, its like they think trash talking isnt part of pvp and why we do it and thats just toxic thinking" which just goes to show how there are some players that forget this is a japanese mmo. It is not our culture of "fuck you asshole you are all here to serve me and facilitate MY experience" and it was more evident with all the in game lines and ques during the beta and early 2.0 but still around. The douchebag culture that creates 'that guy' in western produced games is not as big a thing here and is absolutely not tolerated at all. Call them out on it rather than get in a shitflinging match and most either learn some manners or just go "that games full of whiny babies" and go back to whatever cesspool they came from.
Unless its Balmung of course, thats a swamp for the duration now