First get Ned's rifle, now keep your eye's on the bear it rears up on its hind legs just before it charges, switch to Ned's rifle (it will equip automatically) and shoot it in the stomach while it's on its hind legs when it charges get out of the way, fast. It's an easy fight but if you get hurt too badly remember to use your flask to heal.
Crude & The Gang
Simple enough quick-draw from the start aim for Crude's head and get some easy shots take out your rifle and pound Crude with it be careful not to let your health run low. After Crude has fallen you take out the guys on the ground then take out the guys that where throwing whiskey bombs at you on the roofs.
This can be hard. After shooting all the arrows and killing the rushing native Americans Quick Killer will appear. Simply switch to the shotgun and run around in a circle. He should give chase but always be out of range. Plug him with the shotgun and he'll go down without you even being hit.
Webb & Rudabaugh
Draw your rifle and hide behind the left hand side of the barn you should be able to see Rudabaugh shoot him enough times and he'll go down hopefully without you taking too much damage. Next stay in the barn and wait for Webb to rush past on the horse try to kill the horse if you can as once he's of it, it becomes a lot easier to kill him.
Hoodoo & His Goons
After coming out of the underground quick-draw all the goons around you and then which to rifle to pug at Hoodoo eventually he'll run off. The place should be on fire but now but hopefully you wouldn't have be hurt by it but a much of his mean would have, make your way up stairs taking out anybody in your way before going up the final set of stairs which to Shotgun, after a short cut scene it will be you and Hoodoo hopefully if you hit him enough times down stairs he'll be in the red so a few shotgun blasts or a couple rounds in quick-draw will finish him off for good.
First equip your shotgun it's the only thing worth using and try to get in a position that once you've shot him he'll back away and then come back the same way leaving him open to another shotgun blast and repeat until he's dead. Careful about you being shot as his gun is very powerful. After this he'll run away and load himself with dynamite, hopefully you've got some shotgun bullets left if not use your rifle head for the little gully and blast at him before he can come near you and he'll explode and that's the end of that chapter.
Now the most important thing to remember about Reed is that he heals himself! If you've got any shotgun bullets use then and then follow it up with round after round of rifle and use quick-draw whenever you can. You have follow Reed all around the battle area so try to get on a horse or two this will also help keep your health up. His gun isn't very powerful but he shoot's a lot so make sure to keep your health up and there should be some whiskey bottles lying around the place.
Dutchie & The Riders
Quick draw all the rifle men nest to Dutchie and on the train. Ride down the train out of the way of the cannon and use your rifle to pick off the riders that are coming from that way then edge your way along the train so you can see Dutchie but he can't fire at you and then finish him out with either a Sharpshooter or a rifle.
This is a hard battle. First off, you can't hurt him with bullets--the only use they have is if you shoot him in the face he staggers a big. Equip your newest weapon the Dynamite loaded arrow (thrown dynamite also works but you don't have a large supply of those). You see those cracks in the ground with the gas coming up from them, they blow up if you have some dynamite near it. The trick is to avoid Magruder's shots while being able to shoot at the vents just before he steps on it to make it explode this will hurt him thankfully. Spend about half-an-hour doing this (trust me it takes a while to get your rhythm right) and Magruder will run off and hide up on the face of the wall and start throwing dynamite at you. What you do here is with a snipe or a rifle hit him in the head now run towards him while he throws a bundle of dynamite sticks and you use your quick-draw and aim at the bundle do this a few times to start a cave-in. Be careful not to act like a wimp in this second stage or some of Magruder's men will come up behind you and start shooting you.
Well that's the last boss and the end of the story, you will appear outside the mines now where you can spend your time doing, well, whatever.