WARRIORS OROCHI 3 Ultimate

This is a very easy achievement to get. You can use the craft mechanic by going to the weapon smith in the camp. You can craft either the "X" weapons which are by far the best weapons for any character or orbs that are used to upgrade your weapons with different weapon abilities. To craft those things you need materials which can be found by playing the gauntlet mode. So just start that up when you feel you're ready and once in a mission, keep defeating named enemies, officers, opening chests and picking up the items they drop. After just the first mission you should have enough to craft something and "pop" goes your achievement.

There is also another achievement that requires you to craft 100 times but if you're going for the other gauntlet mode related achievements, you should naturally collect enough materials to get that.