flame him back even harder. pinpoint his insecurities and strike at them, and know that even though he'll lash out even harder he'll do so with tears in his eyes.
or just mute/ignore him and be happy that you don't look dumb nor angsty online. insulting someone shouldn't be classified as cyberbullying though, just as you wouldn't classify someone insulting you bullying.
leaving the game/walking away from/unplugging your computer is also a way to deal with it, because if you can't handle being insulted online, you probably shouldn't be online. the world is harsh buddy, and if this wasn't discussion people would probably be flaming you openly right now for making this thread.