Thunderstone: You can throw this stone as a ranged attack with a range increment of 20 feet. When it strikes a hard surface (or is struck hard), it creates a deafening bang that is treated as a sonic attack. Each Creature within a 10-foot-radius spread must make a DC 15 Fortitude save or be deafened for 1 hour. A deafened Creature, in addition to the obvious effects, takes a –4 Penalty on Initiative and has a 20% chance to miscast and lose any spell with a verbal component that it tries to cast.

Since you don't need to hit a specific target, you can simply aim at a particular 5-foot square. Treat the target square as AC 5.