Ratchet & Clank: Up Your Arsenal Hints

If you want to unlock the infector bomb weapon quickly, go to planet Mylon. At the beginning there are two big tyhhranoids. Take out your infector weapon and keep shooting them to build your weapon experience. When the enemies are dead, or if you're out of ammo, die and return to the same area to keep infecting the same enemy. Your weapon should level up in no time.