Credit to Shiftie on x360a
Although technically missable, this achievement requires you to find all of the weapons in the game, as well as all of their upgraded versions. You will find the first of each weapon planted in a corpse atop an altar pretty much right in the middle of your path. The upgraded versions are somewhat missable, just be sure to go through all of the branching paths that you come across, but they are also found on similar altars with corpses (except the crossbows, which are just sitting on an altar all alone.)
-The hammer (Bonecrusher) is unlock about halfway through the first zone.
-The Boneclaws are unlocked shortly after the first boss.
-The Plaguebringer (Sword 2) and the Marsh Stalker (Crossbow 2) are both found very early on in the marshes.
-The Tainted Hammer (Hammer 2, duh) is found a little bit later in the marshes, in the area just after Crom falls to his knees and has a vision, look to your right after the cutscene and you'll see the white hammer smashed into a corpse (enemies will spawn, so fight them off first).
-Pestilence (Claws 2) will be right in front of you after going through the door in the marshes that required you to kick down two glowing pillars (required to progress).
-Forsaken Blade (Sword 3) can be found immediately after your first fight in the Forsaken Lands. You'll walk right by it leaving the large area to continue onward, just look to your right.
-Abyss Hammer (Hammer 3) can be found after the second fight in the area, when leaving after the fight, head straight toward the wall (you should see the hammer) instead of going down the path to your right.
-The Void Hunter (Crossbow 3) can be found at the beginning of the floating platform area. Once you reach this area, take a left and you will soon come to it before the path turns back to the right.
-Demon's Gauntlet (Claws 3, and the achievement) is found near the end of this same platform area, once you reach a fork with two fat shielded enemies, go all the way right and you'll find these babies ( you can see it from where you fight the fat guys), while you're here, note that going left instead of right leads to a mana upgrade.