You’ll start to see Superior weapons dropping from level 16 onwards. These are purple rarity weapons (the only tier above that are orange Legendary weapons) with a single mod and three perk slots, as well as one locked, innate perk. Find any workbench, fit a mod of your choosing - impact, puncture, shock, fire, or caustic - then add any three perks you fancy. Assuming you have the requisite materials and the cash, you can deck out your chosen Superior weapon, and pop this achievement.