I ran into trouble with this spell yesterday. Our half elf blood mage used 4 WP and rolled two successes for a total power level of 7. The demon had stat=4 skill=3 gear=2 (an amulet) for the INSIGHT roll. Our interpretation of the spell, as written, is that it lowers skill dice (because that is the pool affected by negative modifiers). So... The demon rolled with stat=4 skill=-4 gear=2, scored 2 stat successes and 1 skill negative "success" for a total of 1 success. Because that is better than the blood mage's zero skill successes (he cast a spell, not a skill), the demon evaded the attempt.
But this doesn't seem to make any sense at all. Should we have treated the power level as 7 skill successes? Or should the mage have rolled for MANIPULATION separately?