Simpsons game help!!!?

I'm at the planetarium and the objective is to Knock Out Jimbo. Bart's shooting him, and Jimbo's shooting back with green stuff. Bart hits him a few times as the elevator is moving Bart to his level, and Jimbo just kind of moans when hit and is unable to shoot back but he does not die. Bart's bullets runs out after a while so I can't shoot for too long. I've been shooting at Jimbo for 10 minutes on and off now. Am I missing something? How do I get past this?