Originally Posted by Darkxid3
Just did that gateway on insane, and it wasnt the last one, so i did the next one on casual and that was the last one instead, so now i have to do act 3 all over again, major annoyed.
You must have followed a different mission path than I did. My party and I got the achievement after beating that mission.
Chapter 3's mission layout is confusing as hell. I would recommend using this guide for Chapter 3 to follow mission flow, to get to the Gateway finale:
Sorry about that. I didn't realize the pathways branched so much. The guide has been revised to include this link and changes to advice.