Simply have a rainbow item equipped in every slot (except class mod) at once to unlock the achievement. You only need to find one gun if you have someone to help you dupe it to make 4 of them. Since there are more than one of everything (but not many), I’ll list all of them, but I will highlight what I felt were the easier items to farm by putting asterisks next to them like this**. Different slots are tied to doing different things in the DLC. Contrary to popular belief, Haderax does not drop every Rainbow item. There seems to be quite a bit of confusion about where to find these Rainbow rarity items.
