Batman notices an open barrel of chemicals a short distance away. Though he cannot read the entire formula written on the label, he guesses it is a barrel of solution used in treating candle wicks. If it is indeed that solution, it is extremely explosive when exposed to heat.
Batman notices a shaft of sunlight shining through a hole in the ceiling. By shifting his position, he is able to reflect that sunlight off his highly polished belt buckle and direct it into the vat of solution. "Prepare yourself for the shock!" Batman yells to Robin.
The resulting explosion blows them clear but knocks them unconcious. Riddler, who doesn't know why the explosion occurred, sees their prone bodies and believes them dead.