This is now a LOT easier with The Taken King expansion! You can now get a Legendary through completing quest lines, available in the Tower. Choose one that has a purple reward, complete that quest to gain said reward, then level up the gun via gaining XP. You no longer need Etheric Light to max level the final upgrade, but you'll still need Glimmer, Weapon Parts and between 20-25 of Spinmetal/Helium Filaments/Spirit Bloom/Relic Iron.