Red Faction: Guerrilla

Another easy way to rack up sledgehammer kills is to take advantage of the enemy AI. They tend to like taking cover, this game is based around destroying cover. If you see an EDF hide behind a wall, slam the wall with your hammer and you'll take out the soldier behind it in the same swing. Achievement comes faster than you'll know it.

Change the Difficulty to Casual, roam around the EDF buildings and swing away with the sledgehammer. The bullets don't hurt you enough to get you anywhere near death on casual difficulty, so you can just charge through them all and wipe them out one by one.

Pretty easy achievement that will come easily but just some helpful information.

I got it on casual, as mentioned in other solutions you shouldn't have any trouble with EDF shooting you unless they are in a vehicle turret. dispatch the gunner with your rifle and the driver should step out making him a sitting duck for your hammer for a second or so. When in the heat of battle just hold down the sprint button (RB) and run around in a zig zag pattern and they wont hit you much and just run up to every EDF and swing your hammer with (LT) or (RT). I find that if they are in cover or next to a wall (RT) works a little better as it comes straight down on them and thus walls and other stuff wont block or interupt your swing.

I didn't go out of my way to hunt EDF as I played. I simply played though the campaign as I normally would except I tried to exclusively use the Hammer to kill all the EDF forces. In general the beginning of the game is easier so it makes this fairly easy not to mention you don't have a huge selection of weapons at this point anyway. The one situation where the sledgehammer doesn't work as well is for the vehicle turrets mentioned before.

The achievement unlocked for me around the first couple of EDF buildings I brought down shortly after entering Dust. You can easily keep track of your progress by pressing Start then select Statistics. from this page go a few pages to the right or left until you are on the screen that has "favorite weapons" along with "kills" the exact number you have killed will be displayed next to the sledgehammer if this is the only weapon you have really used to this point it should be on top of the list. The achievement came up as soon as I hit 100