Follow Rudabaugh and Webb to the tavern, and once you get there a cut scene will occur and you will run into some trouble, kill all of the guys shooting at you and another cut scene will occur. Now get onto the horse and follow the rebel. Be careful of the incoming enemies on horseback and once you get to the ranch there will be a few enemies on foot so take care of them. After another cut scene you will find yourself inside the barn, now it's time to kill Webb and Rudabaugh. Rudabaugh has a shotgun on foot, so be careful. Take cover by one of the stables and use quick draw to shoot him in the head, if that doesn't kill him finish him off with your rifle. Now kill Webb who is on horseback, use the hay bales outside as cover and shoot him as he rides past. A rifle or shotgun well aimed can deal some serious damage to him. Once they are both dead the achievement will unlock.