How about a new reagent. It would be an 'addition' to a spell
using any other reagent, and it would 'penetrate' the target's
resistance. In example. A monster resists all types a bit too much,
or just the types the unlucky mage has. The mage could use a
compound mixing the spell reagent, and an enhancing element
that would lower the monsters resistance to that particular spell.
Once, of course. If the spell is cast again without the compound
the resistance would naturally be as it was.
Of course, the effect of the element would be equivalent to it's
quality. Aaand these elements would cost a mint.