InFamous - Grenade Launcher wielding Dustmen? (PS3)?

Hey people, I'm trying to do the f**king annoying stunt, 'Right back at ya'...I'm on the 3rd Island now..does anyone know where I can find some dustmen with grenade launchers (if theres on in a side mission please name it) on the 2nd Island? if not,any tips on doing this stunt? thanks..