Cook chicken in sauce:
Arrange the chicken breasts in a single layer at the base of a 6-quart slow cooker. Drizzle 1 cup (10 oz) of enchilada sauce over the chicken. Secure the lid and set to LOW for 4-6 hours or HIGH for 2-3 hours, until the chicken is fully cooked and easily pulls apart.Shred and return chicken:
Transfer the cooked chicken to a plate and shred with two forks. Discard any remaining liquid from the slow cooker. Return the shredded chicken to the pot.Mix in remaining ingredients:
Incorporate the softened cream cheese, drained black beans, drained corn, undrained green chiles, and 1.5 cups of shredded cheese into the slow cooker. Mix thoroughly until the cream cheese is fully integrated and the mixture is uniform.Fold in tortillas and sauce:
Fold in half of the torn tortilla pieces until evenly distributed. Pour the remaining 1 cup (10 oz) of enchilada sauce over the top and fold gently to combine.Top with tortillas and cheese:
Layer the remaining tortilla pieces on top and sprinkle with the remaining 1.5 cups of shredded cheese. Cover and cook on LOW for 30-45 minutes, until the cheese is completely melted and the casserole is bubbling.Rest and garnish:
Allow the casserole to sit for a few minutes before serving. Finish with desired toppings such as sour cream, cilantro, or avocado.