Here is a sprite I have taken from internet and trying to slice it out in many smaller pieces.
But the problem is I have polygon shape sprite, how to cut it out, selected in picture ?
Rest are just fine.
Why do you need to do this? Just move the top right part in a programm like photoshop or gimp and then let Unity slice it automatically. If you later need collision detection on that ground, you can create an edge collider.
I don't think there is way to slice something into a non-rectangle form. Otherwise please correct me.