This is quite a hard mission, so be ready for a long haul. Grab a heavily armoured vehicle (the Stampede) and rush in and take out all the guards in the first stage. The car should be able to take all the damage, but if you have to get out of the car and you do get hurt, there is a medical pack near the start. Once finished dealing with them all (watch out for the guy hiding in the train cart), park the car away from the entrance of the garage and enter it, heading up to the highlighted van. A driver will now appear. You need to protect him from the newly spawned guys, which is a pretty simple job. There is another medical pack just inside the garage if you missed it.
Take out the guys coming through the train cart and carry on in a crouch. Keep low and take out the few guys hanging around. Once you go far enough, a whole horde will spawn just the other side of the train carts near the building. Go around and take them all out in one fell swoop. Now take out the guys hiding behind the water trailer. Note that there is a health pack on the side just in case you need it, but if you were careful, you shouldn't be hurt at this point. Now you should reach a building full of guys with AK-47s, and they'll have vantage points everywhere, taking shots at you. To top it off, there are oil drums scattered around the place. Enter blind rage mode as you go in and head up the stairs to the right. It's important to get up onto the balconies, or it'll become a lot harder. Go nuts in blind rage and just clear out the few remaining after that. Remember to backtrack to health packs if you need to. Once you've got through that, get to the second van and protect the second driver.
Now go find a heavily armoured vehicle. There are a few lorries around so you could borrow one of those if you wanted, or get your own car. Either way, head on to the other side of the road and prepare to set loose the dogs of war. Take out most of the guys outside driving around, but try not to drive into the garage, because they'll shoot it up and chances are you'll either get caught in the explosion or caught in a barrage of bullets. Keep killing all of his men and once you enter the garage (on foot), go into blind rage and clear it out until you get a cut scene. The door to Nacho's lieutenant will now be open, so enter it. He'll play a little game of hide and seek, with him staying behind a screen most of the time, then periodically coming out and attacking you. That's your chance to lay into him with explosives or machineguns and blind rage is good at this point. He'll also have henchmen come in, so kill them and build up your rage meter. Stay low under the desk so you don't get hurt. After a while you'll kill him.