Unwavering Achievement in Mass Effect 3
Finish all multiplayer maps on Gold or all single-player missions on Insanity.
How to unlock Unwavering
SP: There are a total of 27 missions, one of which (Priority: Earth) never shows up in your quest log. This achievement will unlock while going for your Insanity playthrough achievement, just be diligent that you complete side-quests as they are available because some are time-sensitive and can be failed (if you complete a "Priority" mission first). The full list of required quests can be found HERE, including instructions for when they open/close.
MP: You just have to beat each of the 6 maps on Gold (hardest difficulty) by finishing all ten waves. This is fairly difficult, so try to find a good team with communication and have high-level characters with a range of abilities.