Re: cancelling out the source of their mystic power
In principle, one requires a certain number of gems to cast each spell. Each spell defines the number of gems required to cast it (I'm converting this from a strategy game I liked so I'm pulling the information from material associated with such). If you don't have the correct number of gems to fuel the spell, you don't get to cast it (so far).
By design, casters use gems to fuel their spells. The gems are usually associated with one of the four elements or light or darkness. The object of this particular spell is to reduce the power supply, not necessarily to negate a spell, hence my dilema.