Warhammer 40000 Space Marine

Note: Part 1 includes Chapters 1 - 4. It is not, as some have mistakenly believed, only Chapter 1. The achievement will pop as shortly after you destroy the drop ship at the end of Chapter 4.

Easier than you'd think. I would suggest NOT doing this your first time through the game. Instead, first play through the entire game on Hard difficulty. Make sure you've completed the game before using the "select mission" option or starting a new campaign, or you won't get the achievement for beating the entire game on Hard. After you've beaten the entire game on hard, come back and go for this achievement. You'll find it much easier as you've gotten used to the controls and strategies for when you're swarmed.

Once you've played through Part 1 on hard to get a feel for the way the monsters spawn, you can sprint through the entire thing in about an hour and a half. Make sure not to get too reckless, though, as you will die :P I got this on my first try after completing the game on Hard.

General strategy: Kill Shoota Boyz (the dudes with the guns) first, because as long as you aren't attacking them, the Slugga Boyz (the Orks with the axes) will usually focus on your companions. Always save your Fury for when you're low on health, as using it to recharge is much safer than trying to execute when there's a ton of other guys around. Don't save it too long though. Remember, it's not instant heal, so if you're too low and still taking damage it won't save you.

There are really only three trouble spots.

Although I did not find this part to be hard, I added this for those who may. Thus, I will not consider it one of my "trouble spots" - and you will see why in a moment. At the end of Chapter 1 (the tutorial) you will find yourself mobbed by Orks if you go towards where the turret is. Instead, don't rush out and fight them. Stay behind cover, and don't proceed into the final area. Once you blow open the pipes blocking your path, use them as your cover and stay just behind them. You should be able to kill any of the Sluggas before they get to you with your bolter pistol, and the Shootas won't be able to hit you since you are behind the pipes. Once all the Sluggas are dead, you can pick off the Shootas one at a time from the safety of the pipes, making sure only the one you are killing has a line of sight on your position.

Chapter 2:
Towards the middle of Chapter 2, right before you enter the bunker to meet Lt. Mira will be a HUGE swarm of Orks with no real cover areas. Hang back behind the defensive area the NPCs are all guarding, and let most of them do the work. Shoot with your bolter at the Shoota Boyz as they will cause you the most trouble. After about 3 waves of bad guys, a drop ship will fly in an Ork Nob. The easiest way to defeat him (since AirNath pointed out you don't have fury yet) is to use your bolter and roll. I remember my first time through this part (before the achievement), I was very low on health and actually just kept rolling away from him and let my companions do the killing. That is always an option if you are in danger.

Also in Chapter 2, when you have your Jump Pack don't go over-board. I almost died cause I was having too much fun ground pounding everything, and was getting owned by the rocket launchers. If your shields drop, remember to wait somewhere safe where the rockets can't hit you.

Chapter 3:
At the very end when Leandros is pushing in the shell. This area can be hard if you don't know where to hide out. As soon as you enter the room, there will be a group of Slugga Boyz trying to dismantle your bomb. As soon as you've killed them, run straight across to the other side of the room. Here there is an alcove where the Shoota Boyz can't hit you. Hide here, and kill everything that comes your way. Don't worry about the guys that come to attack you, instead make sure to kill the little Squigs (the little pink things with bombs strapped to them) first, since they are the most dangerous and will also blow up most of the Sluggas.

Chapter 4:
Make good use of your ranged weapons and you won't have too much trouble here. Once you've gotten onto the battering ram, be sure to move slowly and keep your eyes open as you will find lots of squigs trying to sneak up and blow you to pieces. When you get to the part with the Drop Ship, make sure to rip the turret off as soon as you get to it. Move around a lot, and use the walls for cover. In the middle area is a spot with a metal plate on the side and barrels in front, forming a little corner for you to hide and recover your shields when necessary. Every once in awhile, the drop ship will drop off a couple of the Slugga Boy Pros and a Nob, so watch out for them.

There you go!
If you decide to downvote this, please post a comment as to why so I can update the solution to be more helpful. Thanks!

Having just completed this achievement, I feel it would be beneficial to add this solution. I read and used the excellent guide provided by Worhammer, and I agree with the general strategies listed, but I have an alternate approach that worked for me on the first try, after having failed several attempts at progressing through chapter 3.

While I would agree that this achievement is more attainable during a 2nd playthrough, I do not think it's necessary to complete the game on Hard before going back to this. I completed the game on Normal, then went for this achievement straight away. My intent was to first complete the game on Hard and then return for this achievement, but I changed my mind when I realized that I would learn more from the difficult portions and figure out which weapons I should use during each battle if I just persisted and tried this repeatedly until I was successful. The other potential pitfall to waiting until the game is completed on Hard is that you may become dependent on certain weapons; at the beginning of the game you unlock weapons incrementally and if you think you're going to begin your first encounter with the Melta Gun or Thunder Hammer, then you will be sadly disappointed. It can be frustrating because truly the most difficult bit occurs at approximately an hour into the game (late in Chapter 3) and if you die you have to restart, but hopefully this information will help you to avoid that pitfall.

Chapter 1 - Not bad really. Use the initial battles to learn how to deal with swarms of Orcs, master melee combat in this higher difficulty level, and to hone your Bolt pistol skills which has infinite ammunition. This is important because you will want to conserve ammo for the Bolter rifle for when it is really necessary. Ranged weapons are EXTREMELY important to avoid deaths on Hard mode. If you die in Chapter 1, no big deal, just start a new game and you'll breeze through on the 2nd try.

Chapter 2 - Exactly as described by Worhammer, but I'll expand on a couple parts. When you have the Jump Pack, there is no need to kill all the Orks (even though it is quite fun). Keep moving toward the location for the checkpoint and you will advance beyond most of the enemies and avoid damage and loss of ammo.

For the portion mentioned just before meeting Lt. Mira, when the swarms of Orks arrive, be sure you are positioned just in front of the bunker and behind the NPCs (opposite of where the Orks approach you). They will take out most of the Orks, along with your help, and with the Bolter in hand you can complete this section entirely without ever relying on melee combat, which is much safer. I would also recommend saving a couple grenades for when the Ork Nob attacks.

Chapter 3 - There are tons of areas where weapons are stashed in large crates, and they provide infinite ammo. I suggest loading up at those sites, and after large engagements return to them to refill your weapons. The Vengeance Launcher can be very useful, particularly in dealing with swarms as it thins out the enemy’s ranks in case a few get through and you are forced to use melee combat. Plus, nearly every time you feel you need to use it, there will be an ammo crate nearby for restocking.

The point where Leandros pushes the armed shell into place is the hardest part of this achievement. You will be faced with Shoota Boyz on two elevations as well as multiple lesser Orks and, most concerning, Squigs. I died three times here when I positioned myself deep in the nook at the far left of the room despite being protected from the Shoota Boyz’s gun fire. This was because I had poor visualization of what was approaching me and by the time I could see them, they had over-run my teammates and I was forced into melee combat. ProTip: melee combat vs. Squigs = dead Space Marine!
Instead of burying myself into that alcove, I began right at the front corner. As soon as the Shoota Boyz entered the upper level of the big room, I picked most of them off with the Stalker Bolter (sniper rifle; I suggest two quick shots anywhere on the body since this is much faster than lining up single-shots to the head). Very soon swarms of Orks and Squigs will attack you, and as soon as I heard them coming I abandoned the Stalker Bolter and switched to the Bolter rifle. This is an automatic weapon and mows down Orks, especially when aiming at the head. Try to save Commander’s Fury for this part and when the initial swarm approaches with the Squigs, turn on Commander’s Fury ‘Marksman’ by pressing in both the RS and LS, then LT. This causes the swarm to run in slow motion and you can easily destroy the Squigs and most of the Orks. By the time your Commander’s Fury runs out, run back into the alcove to protect yourself from the respawning Shoota Boyz above, and there may only be 1 or 2 Squigs left which you can dispatch with your Bolter in regular time. Your last challenge at this point will be an Ork Nob who completely ignores your mates and charges you straight away. Do not engage him in melee combat, it’s just too risky. Use your sprint and roll abilities and get some distance from him by running back and forth from the alcove to where you entered the room initially (this provides protection from the Shoota Boyz gunfire) and when he is coming at you, shoot him in the face with your Bolter or blow him up with several shots from your Vengeance Launcher. After he is dead, just pick off the remaining Shoota Boyz from the protection of your alcove and the chapter will end.

Chapter 4 – When you enter the large room with multiple ramps, use your Stalker Bolter to pick off the Orks and occasional Squigs from afar. Once the room seems clear, turn around and restock the ammo in the large weapon crate containing Stalker Bolters. As you advance, more Orks will respawn so just snipe them and backtrack for more ammo if desired. Use Worhammer’s guide for the final portion of the chapter and you are 25G richer. The achievement popped for me at the beginning of Part II so don’t turn off your console too soon.

This is my first guide so I would appreciate any constructive comments. Good luck!