Dynasty Warriors 7

This achievement requires that you unlock every seal on every weapon possible. First, a quick explanation...

A seal is a skill on a weapon that can be learned. Equip the weapon, and then kill stuff. You gain experience for the seal from killing both officers and peons, however you'd need to kill way more peons than officers, so I'd suggest concentrating on officers. When you are looking at a weapon's stats, you'll see the name of a skill in the middle of the stat box, and it'll have a bar below it, this indicates seal progress.

The best way I've found to learn seals is to play arena stages. These are small stages where you only fight officers. You can end them fairly quickly and since you don't have to run around on a horse or deal with peons, it's pretty simple stuff.

You'll need to learn seals off starting weapons all the way up to the "smithing" weapons. The smithing weapons require that you be playing as a character that has learned the smithing skill (Zhuge Liang has this in his tree) and then buy them off the weapons dealer. The merchant also sells weapons including the smithing ones (still need the skill) but at a 20% discount, however he only sells three at a time and he only shows up every 3-4 battles.

There are stages in conquest mode that give weapons, two types. One gives a rare weapon (unique to that stage) and another gives a random weapon you don't have. The rare ones are called Treasure Battle : (insert weapon type here), the others don't have a special name. Regular ones have a plain sword icon on the tile, and rare ones have a sword with um...sparkles around it, like a stupid Twilight bastard, I guess. You'll have to complete all of the Treasure Battles in order to get all the weapons, since they are unique (for the all weapons achievement) and most of them have unique seals.

I'd actually suggest just working on seals while doing other stuff, because there is a shit ton of stuff to do in this game, like maxing bonds with officers/making them sworn allies, learning every skill with all officers to open the voice galleries for the related achievement, and finishing every conquest stage. If you make a conscious effort to use weapons with unlearned seals on them, you'll get this on the way to finishing everything else.