For this Achievement. simply beat All 17 Missions on any Difficulty.
The difficulties are Recruit, Regular, Hardened, and Veteran.
Below you will find a list containing all the missions from Call of Duty: Modern Warfare II's campaign:
1.Strike
2.Kill or Capture
3.Wetwork
4.Tradecraft
5.Borderline
6.Cartel Protection
7.Close Air
8.Hardpoint
9.Recon by Fire
10.Violence and Timing
11.El Sin Nombre
12.Dark Water
13.Alone
14.Prison Break
15.Hindsight
16.Ghost Team
17.Countdown