Insanity Achievement in Mass Effect 3
Finish the game on Insanity without changing difficulty after leaving Earth.
How to unlock Insanity
To make sure you get the achievement, before starting a new game go to Extras from the Main Menu. Go into "Options" and to "Gameplay." Choose Insanity for the Combat Difficulty.
If you started the game already you can still change the difficulty as long as you have not beaten the first mission on Earth. Press and go to "Options". Go into "Gameplay" and you can change the difficulty to Insanity. This must be one before leaving so do it right away. If you find yourself on Mars, you have gone too far and will have to restart.
This mode requires a lot of patience and is best done with power heavy classes (so that you do not run out of ammo) or do the From Ashes DLC first to get the Particle Rifle weapon for unlimited ammo. If you did not import a save and have to play twice anyways, save this for a 2nd playthrough. If you have an import, go ahead and start on this difficulty right away. It's not extremely hard and as long as you save often with full health and keep a backup file, you'll be in good shape.