By Alexandra Alper ZAMORA, Mexico, May 22 (Reuters) - Government security forces killed 42 suspected drug cartel henchmen and suffered one fatality in a firefight in western Mexico on Friday, an official said, one of the bloodiest shootouts in a decade of gang violence wracking the country. National Security Commissioner Monte Alejandro Rubido said one federal policeman died and another was injured in the three hour battle on a ranch just inside the Michoacan state border with Jalisco, home of Guadalajara, Mexico's second-biggest city.