1. Place avocado in a small bowl; mash with a fork. Stir in 2 tablespoons tomato, 1 tablespoon onion, 1 tablespoon juice, 1/4 teaspoon salt, and garlic.
2. Combine remaining 1 cup tomato, 2 tablespoons onion, 1 tablespoon lime juice, 1/4 teaspoon salt, cilantro, and jalapeño; toss well.
3. Combine chicken, remaining 1 tablespoon juice, and paprika; toss well to combine. Spread about 1 tablespoon guacamole over each tostada shell; top each with 1/4 cup chicken mixture and about 2 tablespoons salsa.