Tbh you come across as a dick to me too in that conversation.

The best way to tell someone they're "doing it wrong" is to start with something like

"Do you mind if I give you some tips to maximise your dps?" or "Can I give you some constructive feedback on your rotation?"

People are much more likely to accept help if they see it as help rather than someone making fun of them. If they say no, then screw them since it's DF and you'll probably never see them again. If they say yes then you can help make a player better with your advice.