Simple enough, if a little time consuming. You need to equip your character with epic (purple) or legendary (gold) items in all slots. That means 3 weapon slots and 5 clothing slots. If you are unlucky with the drops you can purchase items you are missing from vendors. I caved in and bought a pistol to get the achievement but the loot in this game is plentiful so you should not need it unless you are impatient like me. This popped for me at level 21 for reference. Please note that you do not need legendary items, epic will do. I had a legendary assault rifle but it was not equipped when this popped for me.