Spoilers Ahead. Really easy achievement to get through natural progression of WotC.
*** Spoiler - click to reveal ***To gain this achievement build a resistance ring in the avenger. Each faction has a chain of covert ops that raises your influence with that faction. Complete the 3 Hunt the "X" covert ops for each faction to raise their reputation to high (Assassin for reapers, warlock for skirmishers, hunter for templars). You do not even have to complete the Chosen's stronghold to raise the influence, but simply unlock it.