Gun

VIDEOS ARE GREAT. Perhaps a clearer explanation of the strategy may help someone.

PART 1) Run around facing him and throw dynamite at the green gas gizers to make them blow up while he is standing next to them. Don't even bother shotting him at this point just keep tossing the dynamite.

If you run out of dynamite there is some the respawns on the ground. If you have the dynamite bow with ammo this will work too. This takes time and patience.

PART 2) Hide either on the left or right size by the outer rocks that are shaped like a U. If you face through the U toward Magruder in the middle part of the U he will shot but not hit you.

Take your rifle and carefully hit him in the head (this can only require 1 shot if you hit him right) . Then wait for him to throw dynamite and use quick draw to blow the dynamite up at the pillars and rocks above him.

Sometimes a few henchmen will spawn behind and above you. Focus on them right away or they will kick your butt. If you take some damage you may want to pick up some respawning whiskey. After that just head back to the sweat spot by the outer rocks.

Once you find the sweat spot by the outer rocks this is pretty straight forward it just takes some time for the rocks to blow up.

Watch the videos!

Part 1: Part 2: Part 3:

Finally, that last mission is here, and the last story related achievement is in your grasp, but be prepared, this mission is loooonggg. The first part of this mission revolves around getting the cannon into the mine and rolling it through in order to get to Magruder, after which you'll have to face him. So, let's start, follow Clay to the cover and there's a few enemies you should be able to take out, do so and then head for the cannon. When there, you'll have to blow up all the attackers which shouldn't be of great difficulty because you have a cannon and they have guns. Shoot all the gunners you see and the cannon will start moving, then blow up the mine entrance to give you a pathway in there and kill the enemies that come running out to greet you.

Now, because the NPC's are pretty dumb you get to do everything (thoughtful right) so dismount the cannon, flick the switch and get back on, continue onwards into the depths of the mines. As you progress, there'll be a few more times which require you to change switches, do so when you need to and don't be afraid to dismount the cannon at other times too if needed due to low health or if you decide that quickdraw or a rifle is better suited to a situation, I was safely able to stay on the cannon for the majority of the killing. Also, make sure that you watch out for enemies mounted on gatling guns as these can whittle away your health extremely fast even on easy, so make these your top priority followed by TNT barrels.

Eventually you'll reach an area where the cannon stops and you have to protect your allies from waves of enemies that come from various directions. At first this may seem daunting but these enemies are weak as always and can easily be killed using quickdraw or just a plain shot, and to your aid there is quite a bit of whisky laying around in this area. After roughly half a dozen waves of enemies, the cannon will be set to get back on course and you can proceed, destroy the blockage up ahead and you'll be left all alone after a cut-scene. From this location you don't have a particularly long way to travel to find Magruder which is handy as some of these enemies can be a pain due to their cover, but just continue towards the marker on the mini-map and before long you'll reach the final battle (well the first half) of the game.

So, here's Magruder and he's impervious to bullets, well that's no fun! At first glance this battle seems quite difficult but it is really quite simple and you only need your trusty dynamite bow for the first half. You'll immediately notice that he throws dynamite at you repeatedly which you should dodge if possible as shooting them leaves you somewhat open. Now, the trick to beating Magruder is that you have to shoot the geysers on the floor (they spew out gas) with the dynamite bow, but you have to get the timing right as he needs to run over them as they explode. To do this aim to hit them about 1-2 seconds before he walks over them and you should hit him fine, rinse and repeat until he moves up to the ledge.

Now, for the second part of the fight you have to destroy the stalagmites by destroying the dynamite when it's close to them, in total this should take 3 successful dynamite explosions. There's a way in which you can essentially make yourself unhittable as the rock that you take cover from will prevent his gun shells from hitting you and if you place yourself correctly you can still hit the dynamite. This rock is located slightly to the right of where you start this section and has a 'V' shaped part to it. If you line yourself up with the said 'V' shape and just walk backwards a little bit you should be able to shoot Magruder (the cursor thing will be red). After you shoot him a couple times he should stagger and then he'll throw his dynamite, as soon as you see the dynamite use quickdraw and destroy it as it's near ceiling height.