[Saint]

Tools and theory.

No spellcaster would be without their instruments and gizmos - and these might not work if they haven't been properly calibrated or knocked about and dropped. Or maybe you can still work your mojo, just at reduced effect.

If you've got a body of scholars that correspond frequently, they may have working theory on the limits of what can be done. "A coherent light attack of that magnitude (a d8 fireball) is impossible. No one has ever achieved greater than magnitude 6 (a d6 fireball) before!"

Whether or not the limits can be exceeded, well that sounds like a good plot hook to me.