To earn this bonus you will first need to wait until you're able to use the bomb shot. Early on you will be prompted to shoot at a score icon with the bomb shot. Once you fire your shot, you will notice that it starts to beep when it's close to a target. To earn the perfect timing bonus you will need to detonate your bomb shot by rasing your arms above your head to activate it.
As long as you detonate the bomb before it hits your target this will unlock.