Important info for Unwavering achievement
This is for anyone who plans to get the Unwavering achievement through the single player campaign.
If for some reason you missed a mission while playing through the game on insanity and did not get the Unwavering achievement along with Insanity, you DO NOT need to play the entire game again on insanity difficulty. Instead, you can track how many missions out of the 27 you have completed through 'extras' menu in the main menu, then try to determine which missions youve accidently missed, then play the game again on insanity only up to that point.
So if you had missed a mission on the first act and then continued to the end of the game, all you would have to do is start a new game and play until youve beaten the specific one you missed. At that point the achievement will unlock as soon as the mission is completed.
I don't know if you have to play the game on insanity before you start that specific mission or if you can switch the difficulty before starting it, but it's always best to play it safe with these kinds of achievements and start your new game with the insanity setting already set.