THIS IS NOW OBTAINABLE AGAIN! (Only available during the holiday season, Every year the dates are a bit different, 2013 it was the 3rd and last year it was the 5th. It is safe to assume that the season starts within the first week of December and ends within the first week of January)
No Longer available.
Easy enough to get the achievement, just kill special infected in Versus and pick up the little green present that they drop.