If you want a light crust like that of tempura, try to fill the batter with air.
One way is to add baking powder. The released carbon dioxide will lighten the batter. (Air is a general term, any gas will do.)
Another way is to use ice cold water (to be confirmed). Cold water (4°C) will dissolve almost twice as much air as room temperature water (26°C) (source), making it twice as crispy.