Resident Evil: Operation Raccoon City

To earn this achievement, the game asks you to kill one-hundred zombies through CQC, or close-quarters combat, in other words.

In order to do CQC, all you have to do is tap B on the control pad. When you are dealing with a zombie, repeateadly tap B in the zombie's direction, constantly slashing and kicking at it until it's down or you can tap B, then hold down A for a gruesome melee kill, which also counts towards the achievement as well. I recommend the latter, since it's faster that way, although you can only do this one at a time, so please keep that in mind!

That being said, throughout the campaign, you will be surrounded by many zombies. Start working on the achievement by killing as many as possible using CQC and eventually, as the game progresses, you will kill your 100th zombie with CQC and the achievement should unlock right after it happens.

If you still need help on this achievement, please check out the video I have attached here. Credit goes to squareonegames for creating and posting this video. The creator of that video also recommends mission seven, as it has a lot of zombies for you to take on. But like I said earlier, I earned this achievement long before I made it to mission seven, since I started working on it since I got the game on release.

Have fun and good luck!

Easily achieved on Mission 4 of the campaign.
You'll come to a point in the mission where you have to collect a large syringe and stick it into the monster guys head to retrieve the parasite. After you retrieve the parasite from the monster, head to the door on the walkway above, here you'll see a cutscene of the parasiteless baddie heading your way.

You'll resume control of your character and have to collect 3 keycards to progress through the door. These are marked by map markers on your Minimap, Collect these 3 keys and return to the door to open it.

Now the fun begins.

You'll be in a room in which you will have to activate 2 switches. While you are in this room there are hundreds of zombies. They continue to respawn until you activate both switches and progress.

There are not infinite zombies here. However there is easily enough to complete this achievement. If not simply die and reload checkpoint.

Simply punch, slash, stab and kick away till the achievement pops. The good thing is the zombies spawn in easily manageable waves and your team mates mop up most of the flankers leaving you free and un-hindered.

Good Luck.

P.S Don't vote down for the sake of it, if there's anything wrong with my solution let me know so I can fix it. Thanks.