The description for this achievement is misleading. All 500 pigs must be frozen by the ice bird when tapping to freeze. I've found two levels with an ice bird that should help.
first is 2-7. You start with an ice bird. I aim the bird between pigs 3 & 4 (if counting left to right) and freeze before hitting any blocks. Immediately restart, rinse and repeat.
second is 3 - 10. Throw first bird away, anywhere you want besides where you'll send the ice bird. Throw the ice bird between the top two pigs in all the ice blocks. If your aim is true, which mine rarely was, you can freeze both. The reason for doing this stage is the number of blocks you can break with the other birds to work towards to block achievements.
I hope these help you and if anyone finds any stages that may help I'd be glad to add them.
Thanks to creeping monte for finding this level: On 2-23 there's a pile of pigs and your first shot is ice. Easily 4-6 pigs each pass. Aim to go around the outer atmosphere and you'll land in them
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