SPOILER ALERT !!!! If you do not wanna know how the game ends DO NOT read any further.
Destiny Defiant : 75gs
Ok this is for defeating the final boss Tirnoch. DO NOT try and attack her directly , you cant touch her and magic spells do not harm her. Honestly this is probably the easiest boss fight i have ever seen. When you come face to face with Tirnoch stay back a bit so her melee dosent hit you. She will zap you and make a fate copy of you. just quickly destroy it and collect some fate. Once you have a full bar activate your fate shift and knock her down. Quickly attack her head until she jumps up and brings you to another platform. Just stay back and destroy the fate copies she creates until your fate shift bar is full again and activate it. then beat on her some more until she gets up. do this about 4-5 times and you while start the final cut scene , the only button press i had to do was rapidly press A for double exp. , the achievement will pop after the credits. But after the credits you are allowed to free roam the worlds and do more secondary quest and finish up any other things you might have going on. But just know enemies are still every where. Hope This Helps..if you are still having trouble with the final boss i found a video. This is not mine and i take no credit for it.