Why are Bomber's friends so weak?
About the "weaklings": Considering how Genthru (the Bomber) wanted to heal his friends first means they're friends and not just allies. So it doesn't matter if they're weak, he wants them by his side because he trusts them.

Gon as of from the start wants the hardest challenge (Hisoka, Pitou), so it makes sense that he takes the leader. Also Genthru is at least high level which means even if Gon and Killua take him head on, he would still win. The reason Gon is the right choice is because of the plan. They need a powerful attack (Janken), enough to go through Genthru's defense. And Killua doesn't have that kind of power.