Like with the kill achievements (Recruit, Soldier and Veteran), this achievement tallies your total kills across all three games, unlocking when you’ve killed 100 enemies with melee attacks. This will obviously be easier to achieve on lower difficulties, as wading into melee combat on higher difficulties can be somewhat suicidal, especially in Mass Effect 2 and 3. If you want to grind this out, use a high-level character in Mass Effect 1 (preferrably on a New Game+ run), lower the difficulty to Casual, and start smacking enemies with your guns. Between the low difficulty and Colossus X armor, this should be a trivial grind.