Make the tentsuyu first. Combine the dashi, soy sauce, mirin, and sugar in a small saucepan and bring just to a simmer over medium heat. Take off the heat and keep warm. Stir in daikon and ginger at serving time.
Prep the shrimp. Make 4 to 5 shallow diagonal cuts across the belly side of each shrimp, about halfway through. Press each shrimp gently flat so it lies straight. Rinse quickly, pat completely dry, season with salt, and dust lightly with the 2 tablespoons flour.
Heat the oil in a heavy pot to 350°F (175°C). A drop of batter should sink halfway, sizzle, and float back up, turning pale gold slowly.
Mix the batter at the last moment. Whisk the cold egg and ice water together, pour into the flour and cornstarch, and stir only a few strokes with chopsticks until just combined. Lumps are fine.
Hold a shrimp by the tail, dip in batter, let excess drip off, and lay gently into the oil. Fry 3 to 4 shrimp per batch.
Fry 2 to 2 1/2 minutes, turning once, until pale golden and crisp and the shrimp is just opaque.
Drain on a wire rack. Let oil return to 350°F between batches and skim loose batter bits.
Repeat with remaining shrimp, mixing fresh batter if the first bowl warms up.
Serve right away with lemon, shiso if using, and small bowls of warm tentsuyu.