You cannot earn this achievement anymore.
The **“Global Thermonuclear 360”** achievement (360 Gamerscore) in Call of Duty: Modern Warfare 2 (2009) was tied to the **“Global Thermonuclear War”** **limited-time event / playlist** (likely a 24–48 hour weekend event in 2009/2010).
Same — achievement locked to event. No mod enables it legitimately (would be flagged).
Focus on obtainable achievements: Campaign Veteran, Spec Ops Stars, Multiplayer ranks. The event ones are historical artifacts.