In a saucepan bring apple juice to a boil and reduce to a sugar syrup. Add beer. Do not foam. Bring to a boil. Lower heat, let reduce by 2/3. Set aside.
In a large bowl combine mango, pineapple, shallots, scallions, plum tomatoes, add cilantro, lime juice, vinegar and oil. Set aside.
In a small bowl mix 1/4 cup of cornstarch and some of ice water make a slurry. In a large mixing bowl combine sugar, flour and remaining cornstarch. In a saucepan heat slurry stirring constantly until slurry is clean and pasty. Remove from heat, add ice water. Stir to combine. Add ice water slurry to flour mixture, whisk to make tempura batter.
Preheat deep fryer 400 degrees F.
In a saucepan heat beer sauce.
Dip soft shell crabs into tempura batter and deep fry until crisp and golden brown. When done remove crab to paper towels, let drain and season with salt and pepper. Arrange mango pineapple salsa in center of a plate. Place soft shell crab on top salsa. Add beer sauce to plate.