Rootin' Tootin' Lootin'G 24

Exceptional weapons are in the yellow/gold category and are the rarest types of weapons that can be found in the game. Most of the time the stats will higher with these but not always which seems odd. You'll most likely earn this achievement just by playing through the game for a while. On rare occasions characters in the story might give you one as a reward for completing a quest but you'll most likely find them a little more often from looting the chests found all throughout the game. You'll want to have the Picklock skill fully unlocked under the Survival Skill Tree since exceptional will mostly be found in level 3 chests (the red ones). This is one of the skills that all 4 characters have and it can be unlocked early on in the game if you choose to (Sam B, Logan, and Xian as early as level 7 and Purna as early as level 4).
Weapon rarities:
White- Common
Green- Uncommon
Blue- Rare
Purple- Unique
Yellow/Orange/Gold- Exceptional