Defending Against Spellcasters, Part IV

4. Hit the spellcaster with some continuing damage (e.g., Melf's Acid Arrow). When taking continuous damage, half the damage is considered to take place while casting a spell. You must make a Concentration check (DC 10 + ½ the damage that the continuous source last dealt + the level of the spell you’re casting). If the last damage dealt was the last damage that the effect could deal then the damage is over, and it does not distract you.

If you don't have Acid Arrow handy, try lighting the enemy's robes on fire with a torch, or hurl a lantern or flask of acid at them.