Pile Up! Box by Box

I found that this achievement had potential to be hard to unlock without a visual guide. In the Forest Fair level (Third Main World) as well as a few in Boxling Square (The Over World) is where you can find the squashes. Most are fairly obvious and tend to only be near where the spiky friends (hedgehogs?) are at, there is however one out of view from the camera and another easy to miss one. The easy to miss one is behind the arrow sign just before the first gate you unlock in the level in the room to the right of the area with the well and the spider in.

The one that I missed and got by just hugging the walls was immediately after the Mother hedgehog moved and to the right of the room down the newly opened path (Before having to use the exploding box to jump up to a ledge)

Also, if you go back into the level, the squashes will have popped back up. You do not need to smash them again, only the ones that you haven't squashed before.