Mass Effect Legendary Edition
127 Achievements
EA Play
Mission Accomplished
ME2: Save humanity throughout the galaxy from certain annihilation
How to unlock the Mission Accomplished achievement in Mass Effect Legendary Edition - Definitive Guide
As simple as it gets; complete the main storyline. This can be done on any difficulty, so you can make it as easy or hard as you want, spec. your Shepard in whatever way is comfortable to you, bring along whichever companion characters each mission, do a speed run through the game or take it easy. Whatever works best for your playstyle is likely to get you through easier.