This solution is not made for just getting this specific achievement. This solution is for getting ALL the weapon achievements in a faster way, without playing through the entire game 8 times for each weapon.
1. Start a new game (preferably starting from a save after you beat the game, aka a game + save file) on the easiest difficulty. You'll see why in #2. Hopefully you already went through the game on easy so you can use a game +, which means you keep all essence and upgraded weapons
2. Do NOT use any of the MELEE weapons at all for as far as you can go through the entire play through. In other words, run past enemies as often as possible. If you get to a point where you have to kill enemies (eg. a boss guy), use projectiles or ninpo only.
You can get as far as chapter 13 by doing this. In chapter 13 there's these egg sacks or whatever that ninpo and projectiles don't work on that you have to get past to continue playing. This is where you start your weapon achievement. You don't have to go all the way to chapter 13, but obviously the further you get the faster you can get all the rest of the weapon achievements. Once you get to the point where you start using your weapon, remeber the save slot that you used before you started using the weapon so you can reload it and start using a different weapon.
Apparently you can use melee weapons on the egg sacks in chapter 13 thanks to a patch on the game.
3. After unlocking the achievement for whatever weapon, reload the save and use the next weapon you need an achievement for.
Guide for bosses w/o melee weapons, written by Madcap182:http://www.xbox360achievements.org/forum/showthread.php?t=76...