Warhammer 40000 Space Marine

You can easily boost this at the very start of the game.

Right when you land on the Kruzer you get swarmed by Gretchins and two Orks. Simple matter of clicking the LStick to charge and press X when close to shoulder charge and pulp the Gretchin. You'll take out something like a half dozen or more with the first charge. Evade roll away, you can kill the Gretchin by rolling into them so be careful, and charge again to pick off the remainder. The Orks take two hits from full health, so charge them twice or soften them up first.

When you've killed them all restart checkpoint and you'll be right back with all the Gretchin.

Rinse and repeat.

Here is a short video i made showing a great place to get this achievement, its the second checkpoint of chapter 14 the enemies never stop spawning so just select this chapter set it to easy and get your chevo on.

The Charge attack is performed by pressing in the left analog stick to run then, while still running, press the attack button. Titus will thrust his shoulder forward and hit anyone who is caught in the way. When I played through the game I didn't even think the charge attack did any damage since it just seems to stun enemies. Turns out it does do a small bit of damage which is what we need to do use. The lower end enemies can take the following number of hits:
- Shoota Boyz: 2 Hits (Thanks to PBreezey for correcting me)
- Boyz (Melee troops with axes): 4 hits
- Gretchin: 1 hit
- Renegades: 2 hits

Clearly the easiest way to get this achievement is to use charges to dispatch the Gretchin. So whenever you see a horde of these little guys coming out at you just charge right at them. More often than not three or four of them will die per rush so long as you aim it properly. Just repeat this throughout the course of the game and you'll get it before reaching the end. If not before then on a second playthrough it should come fairly quickly - you can still charge Gretchin on hard, they die just as easily.

Obviously trying this on the stronger enemies is heavily unadvised. They take far too many hits to kill while killing you incredibly fast in melee. The only enemy that this strategy can sometimes work on is the 'Ard Boys (the shielded ones) - if you keep running past them with each charge attack they can't strike back. But it takes a whole lot of hits to actually bring one down. So stick to Gretchin hunting.

Late in the game you'll start dealing with Renegades. These guys are weak like the Gretchin but tend to stay at range with their guns. If you can safely close the distance then shoulder charges work just as well on them as they do with the Gretchin it just takes a bit more work.

EDIT - Worhammer has a suggestion from the comments below:
Charge up a plasma pistol shot, and hit the Slugga / Shoota Pros. One shot will get them far enough down that a charge attack should kill them. Not reccomended on Hard, though, as they'll swarm you and beat the daylights out of you.

This is a good suggestion especially when you're alone and can control how much damage is being dealt to the enemies.

The best place I have found to do this is in the second checkpoint of chapter 14 a bunch of psykers will open warp gates and an infinite number of Traitor Guardsmen will come through. To do a charge attack you run by clicking the left joystick and pressing x or y. It will kill the Traitor Guardsmen in one or two hits. This is also the spot to work on your 40,000 kills.