As far as the flash, the flash exposure needs to be closer to ambient for the shadow to be less noticeable. The term is fill flash, just enough to fill in the shadow areas of the subject but not be brighter than ambient. A modifier like umbrella as suggested makes the light source larger, thus softening the line between shadow and light. Also flash off camera so the reflection is not right back into the lens.
In the attached snippet, notice that the shadow of the legs to the left is blurred. A beauty dish was used to enlarge the light source. Also notice that the flash is close to ambient in intensity, in this case backlit sunlight thru trees. The sun is lighting the front of the dress and my flash lights the rest. Had I not used flash this shot would be in the recycle bin. Outside and cloudy can be great light but I disagree with silvermax above, there are many times a flash is useful outside.
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