In a large bowl, combine the shredded chicken, shredded cheddar cheese, diced onion, chopped cilantro, ground cumin, chili powder, garlic powder, salt, and black pepper. Mix well until all ingredients are evenly incorporated.
Warm the corn tortillas in a microwave or on a skillet for about 20 seconds each to make them pliable.
Place about 2 tablespoons of the chicken mixture on one end of each tortilla. Roll the tortilla tightly around the filling to form a flauta, securing the seam by tucking it under.
In a large skillet, heat about 1 inch of vegetable oil over medium-high heat until it reaches 350°F (175°C) or until a small piece of tortilla sizzles immediately when placed in the oil.
Carefully place 3 to 4 flautas seam side down into the hot oil. Fry for 3 to 4 minutes, turning occasionally, until all sides are golden brown and crispy.
Using tongs, transfer the cooked flautas to a plate lined with paper towels to drain excess oil. Repeat with the remaining flautas.
Serve the chicken flautas hot with sour cream and guacamole on the side.