I have not a clue what to do. I may be GM, but I'm not as experienced as most of you.

Firstly, nobody is going to leave the RP. That would be unfair for you.
Secondly, I don't think we can kill Serenity with the RPers consent. The actual Teen Titans messed up a lot in the show, but with undeniably almost impossible odds, they were able to win, with no death of them or the villain.
Thirdly, what if this; Superbody somehow (based on what you people have been saying) disactivates/gets rid of the threat of the bomb, Jack fights the team (and yes, we might need to change his power level, or he would instantly die; and most villains in Superhero shows are more powerful than only one Superhero, and it normally takes nad entire team using teamwork to defeat the villain) and something (we'd need to decide this) happens. 
Fourhtly, I'm going to need to appoint a Co-GM

The third thing I said was just an idea, though