1 cup plain flour
1⁄2 cup cornflour
1 pinch baking soda
Just use a tempura batter mix.
Flours with a high sugar content, potato starch, rice flour, tapioca flour, corn flour, give a crunchy batter rather than a chewy batter which a high gluten (wheat) flour gives.
beer
Or soda water. Either way the liquid should be as cold as possible. Prevents activating the gluten. More crunch, less chew.
Don't work the batter too hard.