Prepare crockpot: Spray inside of slow cooker with nonstick cooking spray to prevent sticking.
Add chicken and sauce: Place chicken breasts in bottom of crockpot. Pour enchilada sauce, fire-roasted tomatoes, green chiles, and taco seasoning over chicken. Stir gently to combine.
Cook chicken: Cover and cook on High for 3-4 hours or Low for 4-6 hours until chicken reaches 165°F internal temperature and shreds easily.
Shred chicken: Remove chicken from crockpot and shred with two forks. Return shredded chicken to crockpot and stir into sauce.
Build casserole: Stir in half the shredded cheese (1 cup), black beans, and corn. Mix until evenly distributed.
Add tortillas: Fold in tortilla strips, ensuring they're coated with sauce. They will soften and thicken the casserole.
Top and finish: Sprinkle remaining cheese over top. Add cubed cream cheese if using. Cover and cook on High for 20-30 minutes until cheese is melted and bubbly.
Garnish and serve: Let rest 5 minutes. Garnish with fresh cilantro and serve with desired toppings.