Whenever I play I usually just move to sarcasm, but you need to pick your words carefully otherwise you could be breaking the ToS. Trying to inform a player that they are bad is probably the hardest problem I have faced in this game. Normally in any other game I would just say a certain word a certain combination of words to address the bad player(s), unfortunately, like I said, you would be breaking the ToS, reported, and subsequently banned. I won't even say what word(s) I would use here, because I would also be banned on the forums.

This is something a deal with on the daily whenever I am playing this game. Usually pointing out their gear, inform them they aren't doing aoe, or saying "our dps must be low" - speaking in general should inform player(s) that something is not right.