You can freeze pigs in two ways. 1) Gently popping a pig's bubble that is in space and 2) Using the ice bird's explosion. The second method is the only one that counts towards this achievement and is easy to grind. This achievement is most easily earned on Level 2-23. Fire the your first bird (ice bird) to the large tower containing 6 pigs, arching him through the gravitational fields. Just before he lands, tap the screen to explode the bird, freezing the pigs in the area. You should have no problem getting three or more pigs with a well place shot. As soon as you freeze the pigs with the ice bird, restart the level and repeat the process until this unlocks. This should take 15 minutes or less to grind all 500.