Taking a look at the shield, it is a bit under-dimensionated for the motor in question.

Boosting it using P-CH/N-CH MOSFETs (TO220 + cooling) should not be so difficult. I mean I would somehow know how to do it, even if I don't have this shield here and I could not really draw a good schematic for this.

 

That's exactly what I have done!! I have used 16 tip-122 and tip-127 transisters to boost it up!! And now the shhield and drive can handle the juice but the mmotor can't!!