This is why this game should'nt get a parse

[Fannah]

he told me " i dont see why i should improve since we killed it" the thing is

its not because you feel bad to no clear something that the content will per magic clear itself

Then what's the problem if you cleared it? In a way, he is right. As long as his dps isn't so low he could be considered carried according to the dps needed for the fight (and not to what others bring, as if you're with 3 other expert dps players, then why should you suddenly reach their level when playing, for exemple, Sophia extreme? If he brings enough dps, he can accept to stop his progression here as long as he's not a burden).
If he is so low in the way if 3 other players like him would be here, they would have seen the boss rage, then yea he is too low.