This also applies to The Warrior and Terramorphous The Invincible. Bosses like BNK-3R drop huge piles of loot which often include several purple levels items such as Weapons, Grenade mods and Class Mods. Bosses can be regularly farmed after they are killed in the story by returning to the area they are located in. General: Purple level items in weapon crates on a regular basis but keep an eye out for Boss enemies. These may be considered spoilers by some so be careful. Guide may include spoilers by mentioning good bosses worth repeatedly killing for Purple level gear. What follows is a guide on how you can help speed up the process by looking in particular areas. As obtaining purple level items is down to luck and how generous the game is feeling, there are few sure fire ways of getting these items to drop. Eridium Tech weapons (Dark Pink text colour) count a purple and work towards this achievement (confirmed by Lavindathar - Thanks). Pearlescent are ranked higher in rarity than Orange.
DLC for Borderlands 2 has added Seraph (Pink) and Pearlescent (Cyan) rarity levels. If you are unsure, the rarity of items from least rare to most rare are as follows: White, Green, Blue, Purple, Orange. This achievement requires you to have Purple level rarity (or higher) items equipped in all four weapon slots, and all four gear slots (Grenade, shield, relic, class mod).
