Defending Against Spellcasters, Part III

3. Attempt to grapple the spellcaster. The only spells you can cast while grappling or pinned are those without somatic components and whose material components (if any) you have in hand. Even so, you must make a Concentration check (DC 20 + the level of the spell you’re casting) or lose the spell. This is often a good option, as the prototypical spellcaster isn't a burly type capable of warding off a tackle from your raging barbarian.