Complete Sasquatch's special missions and rescue enough civilians to get Vienna Cho's distress call. Rescue her and she will join your team. Sasquatch will call you to the same locations where you rescued him at first. When found he will have a tactical nuke and a trigger. Drive back with him then listen to what Vienna Cho has to say. Talk to Vienna Cho again and agree that nuking the city is a bad idea. Escort her to the nuke and have her disarm the trigger. Call for extraction via HQ (only possible after rescuing all high value assets). With the next chopper (after a siege), the non-nuclear ending will begin and the achievement will be earned.