*This achievement is currently glitchy*
Katana Slash is the first power up you get during the game and is available from the start. It has a short cooldown, so once a zombie or group of zombies is at low health, spamming this power up should net you a lot of kills. Simply kill 100 zombies with this power and the achievement is yours.
*Now, to start off, I classify this achievement as "glitchy" and not "glitched", because it is still obtainable and is not fully broken. Many people, including myself, have killed a lot more than 100 zombies with this power, just to have the achievement not unlock. It seems however, that it can be gained by making sure you have killed at least 100, and then "sharing" the game via the Charm menu on Windows 8. It doesn't matter which method you choose to share it, but you will need to share it in order for this to unlock for some odd reason. The share button is found in the charm menu between the Search and Start buttons in the Charm bar. To access the Charm bar on your device, swipe from the right hand side of your screen/move mouse to lower right corner, and it should come up. Credit to user "Grobanite" on trueachievements.com for sharing this solution!*