Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked chicken, cream of chicken soup, sour cream, garlic powder, onion powder, cumin, salt, and black pepper. Mix until well combined.
In a 9x13 inch baking dish, spread a thin layer of the chicken mixture on the bottom.
Layer 4 corn tortillas over the chicken mixture, slightly overlapping them.
Spread half of the remaining chicken mixture over the tortillas, then sprinkle with half of the Monterey Jack and cheddar cheese.
Repeat the layering process with the remaining tortillas, chicken mixture, and cheeses.
Top the casserole with the diced green chilies and any leftover cheese.
Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Let the casserole cool for 5-10 minutes before serving. Garnish with fresh cilantro if desired.