'Astonishing' means surprising. For example, "There's an astonishing range of goods on sale". This means that there are far more goods than you would ever expect.
In its literal meaning, 'stun' means to knock someone unconscious. If something is stunning, it means that its impact is like a blow to the head, rendering you unconscious. We mainly use it for appearance, in the sense of 'breathtaking beautiful' :
You look stunning in that dress.
The sunset over the bay is absolutely stunning.
We can also it as an adverb to intensify an adjective e.g. "His wife is stunningly attractive".
This is different from saying "His wife is astonishingly attractive", which means that you didn't expect her to be attractive.