THE BEST DISSES ARE SPECIFIC, NOT LOUD
Volume is the amateur move. What actually lands is detail: the thing only you would know, delivered flat. A line that could be aimed at anyone is aimed at no one, which is why "you fell asleep during your own set" beats any amount of generic insult.
So put something real in the subject box. The more specific the beef, the sharper the writing comes back — and the funnier it is to whoever is listening.