Mass Effect: Legendary Edition (PC) - Achievement - Medal of Honor
ME1: Complete the game on any difficulty
The achievement is fairly self-explanatory - It is achieved by completing the game. This is just the main story - Side mission aren't relevant. As long as you've Located Liara in the Artemis Tau Cluster, completed Feros, Noveria, Virmire and Ilos, you will find yourself on the Citadel during a Reaper / Geth invasion where you will finally be able to face Saren to the end. Once you defeat him, you've completed the game.