Put the RNG in Orange Achievement

As the description says, you need to find a legendary (orange) weapon in the world, and it also needs to be enchanted. "In the wild" just means from an enemy or a chest, and not dropped by another player. This achievement will actually unlock as soon as the weapon appears (as soon as the enemy is killed or the chest is opened).

Unfortunately, this achievement is entirely RNG dependent. You have no option but to hope to get lucky as you play through the game. You can, however, increase your Loot Luck to improve your chances of getting rarer items such as an enchanted legendary item. Refer to Luck Amok (50G) for more information on increasing your Loot Luck. 

If you're going for the completion, you can count on finding orange items while doing your Chaos Chamber runs, so definitely don't worry about this achievement until you've completed the story, all side missions, and reached Chaos Level 20. If your luck is so bad that you still haven't found an enchanted orange item by then, you could farm any boss over and over with the hopes of getting one.