I Wore 'Em DownG 25

This achievement is unlocked by simply being on the winning side of 50 Attrition matches be it IMC or Militia. Attrition in essence is a team deathmatch game type. You can play Attrition matches in the campaign mode which is a mix of Hardpoint Domination and Attrition. You can also just play in the Attrition gametype to work on the 50 wins.