This achievement is not as hard as it may seem. You just need to level up the classes some, and do a few practice runs on each level to learn the right path. They key to getting a 3 star rating is to keep the combo going throughout the majority of the level; you rarely have to combo the entire thing, but often have to hit 90% or so. See my level walkthroughs for more in-depth tips for each level.
These are the classes: Gunslinger (revolver), Ranger (rifle) and Trapper (shotgun). The Ranger and Trapper classes have a revolver secondary, while the Gunslinger class only has the revolver (but allows dual wield).
Level up all classes at least halfway (meaning 3 perks out of 6). They're all useful for one level or another. If you don't want to spend the time, you can skip out on leveling up Gunslinger as that's the least useful class in my opinion. I don't suggest it as the best class to use for any of the levels, but you might feel differently so experiment a little with all the classes.
The Trapper class can clear out any of the run-and-gun missions like Lincoln County or Grand Valley, while the Ranger class excels in long-range fights like Stinking Springs or Guadalupe Canyon. I feel both types of missions are a pain with the Gunslinger class, while they can be pretty easy with the other two.
- Use concentration (RB) whenever you can't find an enemy, or when you need to pull off an ill-timed, desperate reload! Don't use it for anything else. You don't need the bonus points from it to reach 3 stars, so only use it to extend your combo timer. It doesn't seem to work all the time, but sometimes if you enter concentration right as your combo timer ends, it'll revive the timer if you kill someone quickly enough. Worth a shot.
- Remember to quit concentration when you don't need it! You can press RB again to disable it and save your bar.
- Headshots, or any other type of special kill, really aren't important for your final score. They help, sure, but the real key to hitting the 3 star limit is to keep that combo going for as long as possible. A 50x or more will get you 3 stars every time.
- Always use barrels to extend your combo timer. One of the loading screens says that you can shoot bottles and birds too, but I've never seen any of those or tried it. Barrels are good enough.
- Don't bother with runners! Let them get to their favorite spot instead of wasting 5 seconds and 3 bullets trying to get them as they're sprinting. Shoot someone else, or a barrel or something, instead of even trying to get a sprinting runner.
- Melee is a one hit kill, so that's a fantastic way to prolong your combo if you're stuck in a reload with a guy right in your face.
- You can't shoot through falling bodies very efficiently, so don't even try.
01: Lincoln County
Score goal: 105,000
There's not much to this one. Play it a couple of times to learn the route, then just equip the Trapper class and tear ass through it. Kill everyone, use your focus during "slow" parts and sprint as much as possible.
02: Stinking Springs
Score goal: 140,000
Make your way into the house, killing everything in a combo, and using concentration if you need to. Use the rifle for one hit kills. Once you're at the window, remember to use concentration or to shoot out barrels whenever you can't find an enemy to shoot. They use a lot of pesky cover in this level and the difficulty will be in finding a clear shot.
You might have to replay this one a few times due to stragglers not showing up when they're supposed to, or because there's a ton of glitchy cover in this level - you'll clearly see their head sticking out and you'll be aiming right at it, but the bullet just doesn't hit. It's pretty frustrating and I haven't found a good way around it.
03: Gold Rush
Score goal: 170,000
Sprint through the corridors, blasting away at enemies and barrels with your shotgun. Always remember to keep an out for barrels to extend your combo timer. The first bit of the mine is almost like a maze; I used the right path at every turn to end up on the top of the "large" room (where all the other paths converge), then I activated concentration as I ran off the platform (run, don't jump!) there, and fired towards the barrels in the next tunnel. I also blew up the barrels in the room filled with them before I ran in there, then I sprinted through the room (as there's no one left to kill) and activated concentration in the next tunnel, to give myself some time until the next kill. The rest of the level is just a straightforward sprint through corridors.
04: Iron Springs
Score goal: 160,000
This one is a pain. It's hard to describe what to do, but I'll try: start off by filling up your ammo with the box in front of you. Wait for guys to amass in that area and start killing them off, then look up to the rooftop to the right and behind you (use concentration to find it) and take them out. Then turn around 180 degrees and pick off the guys on the ground, while hanging out around the gatling gun. Then turn towards the walkway in the distance and use concentration if you have to - the guys there show up pretty late after you kill off the last bunch of guys on the ground - and take them out. Then the guys on top of the house and on the rightmost side of the walkway. After that, the enemies on the ground will respawn around the same area where you last killed them. If you head over there, with the gatling gun and walkway to your left, you can turn right once you hit a wall and follow that around another right, and then right again back towards the walkway, and around that path is where all the last of the ground troops spawn. Don't miss the ammo box in a corner in one of those turns - it's very visible and hard to miss. After that, there's a bunch of guys up on the middle of the walkway. There are also lots of barrels around the level, so remember to use those whenever you have trouble finding someone and need to extend your combo timer. This one will take quite a few tries to pull off.
05: Bull's Head Saloon
Score goal: 80,000
A fairly easy and straightforward level, especially following the last one. Just walk down the street, turn left and head into the saloon, while killing everyone in your way. Keep an eye on the saloon balcony and roof for a few of the guys; the rest of them are obviously posted in your path and hard to miss. One little trick is that you can run a bit down the first street before shooting anyone, to force some spawns and give yourself some more time between kills. Remember to refill your ammo by the stage coach you pass, and save as much concentration as you can for the inside of the saloon as you'll need it to find enemies before the timer runs out. Just spam that RB as much as you can in there.
06: Apache Territory
Score goal: 120,000
Get ready to get REALLY annoyed by retarded runners, yay! These runners seem to follow a set path and if you show up too soon, they'll run right past you and act confusing as hell! Keep that in mind and rush as hard as you can is really the only strategy here. There are lots of "slow parts" with very few, well-hidden enemies that are pretty much designed to murder your combo, so you need to be constantly sprinting. The hardest part is the tree bridge; I cross it first, by sprinting, then I turn around and kill the guys up on the ledge to extend my combo. In the final room, you can use your shotgun to great effect with the help of concentration and the "devil's shotgun" (no reloads) perk.
07: Grand Valley
Score goal: ~175,000
Shotgun rush! That's really all there is to it. Shoot out barrels to save time and ammo and to get some more points, and remember to save your concentration for the three shotgunners. If you enter concentration and do headshots, he'll go down in just a few shots. The "devil's shotgun" perk (4th perk) makes a big difference and helps a lot on this level, so level up that Trapper class!
Score goal: 220,000
This is the toughest level to simply survive on, as you're constantly under fire on the rooftop, and then you have to enter the bank and have close-range fights with shotgunners and stuff. Don't worry, though, because it's really not that hard to get 3 stars on! All you need is to fully combo the enemies outside and on the rooftops, and all of the enemies inside the bank don't matter. In order to pull off the combo on the roof, you will need to keep your combo going by shooting barrels. What I would do is shoot any visible, stationary enemies and, while I waited for more enemies to show up, I would aim at a barrel and fire just when the timer ran out. If there were enough enemies in the open at that point, I'd kill them and repeat, otherwise I'd find another barrel and wait. If you get lucky, you can kill the dynamite throwing guys before they can toss one at you, but if not you can just walk to the other side of the roof and keep shooting.
09: Missouri Swamps
Score goal: 130,000
Run a lap around the little house you start in, to collect ammo and dynamite, and then just stay up there for the duration of the level. The enemies will first spawn to the left of where you're looking as you start, and then to the right and then the spawns circle the level until they come back to where they first spawned. Just clear out the enemies in each area and once you notice that no more are coming, shoot a barrel and find more enemies in the area to the right of where you were looking. Throw dynamite at shielders and shotgunners if you can, otherwise remember that the revolver is actually better at taking them out than the rifle is (for whatever reason). This can be a tough level, but I actually dropped my combo TWICE and still made 3 stars, so it's pretty forgiving.
10: Guadalupe Canyon
Score goal: 400,000
In my opinion, this is the easiest level in the game. Start the level off by running a few laps around the stage coach, without shooting anyone, to force the ammo in the ammo box to spawn. As soon as you fill up, start circling and looking for targets to shoot. Whenever you can't find anyone to shoot, hit concentration and exit it again as soon as you find your target. Repeat 50 times. You don't need to full combo this one; I think the highest combo is 77x and I had a 71x and ended up with a 600k score. Three stars is only 400k, so you could probably get away with as low as a 50x combo.
(Score goals provided by Cellar Attic.)