Preheat your oven to 400°F (200°C). Arrange the tostada shells on a baking sheet and bake for 5-7 minutes until they are crispy and lightly golden.
In a medium bowl, combine the shredded chicken, refried beans, chili powder, cumin, garlic powder, salt, and black pepper. Mix well until the chicken is evenly coated with the spices.
Spread a layer of the chicken mixture over each tostada shell, about 1/4 cup per shell.
Return the topped tostadas to the oven and bake for an additional 10 minutes to heat through.
Remove from the oven and top each tostada with shredded lettuce, diced tomatoes, shredded cheese, a dollop of sour cream, salsa, and avocado slices.
Garnish with fresh cilantro if desired and serve immediately.