If you have the trial version or the Professional Version of Font Creator you can use a Glyph Transformation on the entire font or on selected glyphs. Tools, Glyph Transformer, Effects, Thin.
See this Tutorial on Working With Transformations for some tips on using transformations.
There is no undo for Transformations, so copy the glyphs first. Then you can paste them back in, and undo/redo to compare before/after results.
The thin transformation suffers from the same limitations as the bold transformation — that is thin strokes will be made thinner by the same amount as thick strokes. You can, however, apply different transformations to horizontal or vertical strokes. If your script font is like Comic Sans — with even stroke weight — it should work well.
To estimate the value you need to use, zoom right in and measure the width of a typical stroke with the measuring tool. If it is, say, 120 funits you would use a horizontal transformation of 10 funits to reduce it 100 funits (10 off each side). If horizontal strokes were, say, only 50 funits, you would use a vertical transformation of 5 to reudce the weight to 40 funits.