El Shaddai: Ascension of the Metatron

As the achievement says, you must steal the weapon from each enemy on the level. This is done by pressing the purify button (LB) when the enemy is stunned, indicated by a blue aura around the enemy. Therefore, to get every weapon, you have to ensure you stun every enemy before you defeat them. To do this, I played on Hard difficulty since I was defeating some enemies without stunning them on Easy and Normal. I also used the Special Armor you get from collecting all of Ishtar's bones to make things easier, but this is not necessary. To ensure I stunned each enemy without killing them, I just kept using the counter attack move (Hold X) over and over, and no other attacks. After a few hits with this, you'll hear armor shatter and get a blue flash indicating they are stunned. Hurry over and steal the weapon before the enemy recovers. If you miss a weapon, I would presume you can reload your last save, but I didn't do this when I got it, so I'm not entirely sure.

You do not have to collect every weapon during the boss/mini-boss fights. Just the ones wielded by the standard enemies.

Finally, make sure you fight all the enemies on the level. There are a couple places where you can proceed through the level without fighting a group of enemies. In particular, when you jump up using one of the green spring platforms through a tiny hole. From there, you can proceed right towards the rest of the level, but if you go left (towards a freeman) you will encounter some enemies. Also, make sure you follow the paths left just before each section of swinging platforms you have to jump on. More enemies are off in those areas too.

Good luck.