For the tempura: To begin, preheat the oil to 375 degrees F, and then begin the batter. In mixing bowl, mix the flour, baking powder, cornstarch and salt together, blending well. Next, add the egg and again mix. Then stir in the soda water and vinegar, and mix for final time. Keep cold until use and stir again before each use.
For the sauce: In a bowl, blend the mayo and wasabi, whisking together well to form a smooth sauce. Hold cold until dressing the cooked shrimp.
For the shrimp: Slice the shrimp from head to tail, forming 2 even pieces; this will yield 46 total pieces. Next, dip and evenly coat 12 shrimp at a time with the tempura batter, and then place in the hot oil. Cook until fluffy and golden brown, 2 to 3 minutes. Remove and repeat the process with the remaining shrimp.
Once all the shrimp are cooked, toss them with the prepared sauce. Remove from the sauce without excess, and finish with lemon juice. Serve.