Guitar, katana, and chainsaw... And has anybody killed that guy with the shotgun in the huntin' shack yet? This guy sucks. What's the easiest way to kill him???
Grab a Mannequin Torso or Small Chainsaw before you go to him. Stand behind one of the displays and wait for six shots. He will always reload after six shots. Run up to him and beat him with the Mannequin Torso or wail on him with the Small Chainsaw. If you have the Small Chainsaw, just keep hitting him. If you have the Manniquin Torso, you might get hit, but you will damage him greatly. If he's not dead yet, wait for another six shots while waiting behind a store display then go beat him some more. It should only take about two rushes with the Mannequin Torso.
Also, I haven't tryed this yet, but I imagine the Hedge Trimmers' charge attack would work good too. Get those from Crislip's or McHandy's Hardware. The Mannequin Torso is everywhere, just destroy the Mannequins. It's an underated weapon.