With the 6.66 update, you can unlock pieces of armor such as the Evil Cultist ones by reaching a certain level or completing challenges to get the piece early as seen in this screenshot.
The earliest EC armor you can unlock by level is the
CULT 1: ACOLYTE torso at
Echelon 4, Level 1. However, you can unlock the Cultist pieces from Echelon 0 via challenges.
(Thanks to Sushi Washi4268 and Elite1111111111 for pointing that out)Equip the armor piece by going to Customization > Customize Character > Any Custom Suit > Armor and then go to the Evil Cultist tab (by pressing
or
to select any piece for your character.
Now jump into any pubic multiplayer match (doesn't manner which mode) and you should get the achievement when you're in the game. You
DO NOT need a full set from the DLC to unlock it. Just one piece will do. Now bring hell (pun intended) on your enemies in style!