Had to think about this and think about the things I would use;
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Super Curves Remastered, Exnem Body Hair, Auto Shape Enhancer, and of course Adobe Photoshop.
To contour the armpit I would use these items. If you didn't want hair I would skip the Exnem and just use Photoshop to darken the area.
Oh, and each picture is linked to 3D-Load, so you can download the items.