This achievement will take a small bit of work on your part but nothing too challenging. Throughout the game you will randomly find possessed weapons. I found my first one early on in a chest while exploring the various side areas on the way to The Cauldron.
Once you have a possessed item, you have to "feed" it to Level it up. This is done by sacrificing other items to the possessed weapon, destroying those items in the process.
You can accomplish this by clicking A on the possessed weapon and selecting "Upgrade". Once you sacrifice enough items to level the weapon, the achievement will unlock. Note that different items have different values that apply to leveling an item (as indicated by the level bar which appears to the right of the item on the upgrade screen).