I won't say this achievement is easy but the checkpoints in the game are very favorable and therefore they keep the frustration in check... but if you're like me you will have a few moments were you'll want to toss a controller.
I don't have a guide for every encounter, nor do I think the game really warrants one, but I will offer some helpful hints for your purging of the xeno scum. (All very much spoiler free.)
They advertise this game as a melee first, one in where you wade into combat and kill everything, and for the most part they are right but on hard... Well, the game has to change.
Note: The extreme range weapon mentality is really for the second half of the game
Edit/Note 2: While doing a run through on Hard you cannot restart a prior mission... So do not go back for missed servo-skulls till after you finish the game.
Range first; This goes for your weapon selection, to how you approach combat, and what enemies you take out first... As I said you will not survive many battles running head first into them and you will survive even fewer melee (even if you are execution your foes for health) if there are enemies with range weapons shooting into melee.
So if you have a horde approaching of melee enemies back off and move to put cover between you and the range enemies. (It needn’t top be much cover to confuse the enemy.)
Blind Fire into Mobs; as mobs approaches blind fire into them. Early on you can use the Stalker but the best bet is your normal bolter.
Reload All Weapons; Simple enough but reload your weapons after each combat. Double check them. Also try to get a reload before the mob gets to you. You'll never know when you'll need to be able to cut lose with your bolter.
Emperor's Fury, an invulnerable tool don' forget to use it... and not just for combat as you recover health when its activated and sometimes you're better off using it to survive to the next check point than not. (Note: It last longer and regenerates quicker as you pick up more purity seals.)
Weapons; I got rid of the grenade launcher early on... Triggering the grenade at the right time was just really hard to do when you are getting swarmed... and truth be told. I never saw it do allot of damage.
Anyhow, my swappable weapons of choice are the Stalker Pattern Bolter and the Las Cannon.
Note: Its entirely possible to have ANY two of the swappable weapons even though at times it seems to now allow this. Anyhow, if you select the weapon you do not want (like the Melta gun) it will let you swap it for the Las Cannon even if you already have the Stalker equipped. (Thanks to Rhyolitic for pointing this out!)
Early on you're likely better of with the Stalker if it has allot of ammo but towards the end of the game you'll need the Las Cannon. (Its just more cost effective)
In the other slot, truth be told, I kept the Melta around for most of the game as its an amazing 'shotgun' type of weapon (aka use up close) but swap it out when you get the Storm Bolter. The bolter has a bit more range. (and allot more ammo.)
Plasma Pistol/Bolt Pistol; You have unlimited ammo with it... If your reserves are getting low and you have pathetic enemies around let lose with it.
Melee; The hammer is nice but keep to the power axe... In the last mission you get the hammer use it till you can swap for the power axe. (You are lock out of your two swappable range weapon types will use the hammer.)
Ammo Crates: Always be on the look out for them... and when you come to a cache with the various weapons laying always 'pick up' the weapons you are carrying first and then use the general ammo. You will get more ammo this way.
Enemies, All range weapon enemies have the chance to make your life hell but these are the other ones you have to worry about.
Squigs, (Dog like thing) maybe the most dangerous enemies in the game... but also one of the most useful. Always kill them first but try to shoot them when they are next to others. (Thanks goes to Beez 808 for pointing out the creature's proper name.)
Nobz; (Big giant Orks) Can be a serious pain... Don't hesitate to go into fury mode on them.
'Ard Boyz, (They look like they are using armored vehicle doors for shields) See above but do realize a properly placed grenade (aka behind them) will do them seriously harm or knock them off balance for proper melee attacks.
Space Marines; royal pains... regular bolter rounds while peaking around cover will kill them but two shots with a Las Cannon is your best bet. (Aim for the head when it kneels down.) Or if you got the Melta around one shot will set them up for an execution much of the time.
Chaos Champion Space Marine; Unholy pains. Honestly, I'm not sure how many I killed without the Emperor's Fury helping me but it likely wasn't many. They can take multiple Melta shots up close and multiple Las Cannon shots from afar. Luckily, you rarely see them.
Traitor Guardsmen; not rally tough, unless you get a bunch of them together. They can sherd shields quickly. These guys are the gretchins of chaos. If you can get close you can just execute them with the press of a button.
Demons; back off and melee them. They can jump around and avoid gun fire... but sometimes its best to put rounds in them as they approach. (late in the game they are the easiest source for health.)
Is pretty brutal. Hide behind the pillars in the back, you will need to back off after the cut scene, and take the following weapons with you. Storm Bolter, Melta Gun, and power axe. (Yes, you will be fighting for your life in close combat and too much melee will get ya killed.)
Anyhow, hope that helps. I'll update if anyone has questions. (and remember! One checkpoint at a time.)