Do this
Chill the water and egg with ice. Sift the flour on top. Stir with chopsticks 10 to 15 times, then stop.
Why it works
Cold water and short mixing keep gluten low. The lumps are a sign the batter is still weak. Weak batter fries thin and crisp.
When to skip it
Skip it for katsu and karaage coatings. Those use breadcrumbs or starch, not a wet batter.