Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the shredded chicken, salsa, drained corn, rinsed black beans, garlic powder, salt, and cumin; stir until everything is evenly mixed.
Spread the chicken mixture evenly into a large casserole dish.
Sprinkle the shredded Colby Jack (or Mexican blend) cheese evenly over the top.
Bake at 350°F for 20–25 minutes, or until the cheese is melted and the casserole is heated through.
Remove from the oven and spoon on sour cream and guacamole, then sprinkle with sliced green onions; serve warm.