tintmylf324,769 21 May 2016 21 May 2016 21 May 2016
In trap mode, a flying pink pig will occasionally cross the scene in the background, flying from left-to-right or right-to-left (two each direction). To get this achievement you have to shoot all 4 in a single game. Not too hard if you keep an eye out for them.
Note that the achievement will not unlock until you EXIT the game, so don't be concerned if it doesn't immediately pop upon shooting the fourth pig.