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 and beans.
Preheat your oven to 350°F (175°C). Arrange the tostada shells on a baking sheet and bake for about 5-7 minutes, or until they are warm and slightly crisp.
Remove the tostada shells from the oven. Spoon a generous amount of the chicken mixture onto each tostada shell, spreading it evenly.
Top each tostada with shredded lettuce, diced tomatoes, and shredded cheese.
Return the assembled tostada stacks to the oven and bake for an additional 5-10 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool for a minute. Drizzle sour cream over the top and garnish with chopped cilantro and avocado slices if using. Serve with lime wedges on the side.