Skunked!

Skunked is a Multiplayer Achievement in Halo: Reach. This achievement is awarded when a player wins a game of Invasion in the first phase.

Walkthrough
Winning a game in the first phase is only possible while defending in a standard Invasion match, not Invasion Slayer. To unlock the achievements players must successfully defend the two marked territories in phase one on Boneyard, Breakpoint or Spire.