Season the chicken generously with salt and pepper.
Heat a Dutch oven or a heavy-bottomed pot over medium heat. Add 2 tablespoons of oil and the chicken, cooking and tossing until browned and fully cooked, 6 to 7 minutes. Set aside on a plate.
To the same pot, add the remaining oil. Add the onion and bell pepper, cooking until the onion has softened, 4 to 5 minutes.
While the vegetables are browning, shred the chicken with two forks or clean hands, then set aside.
To the onion and bell pepper, add the chili powder, garlic powder, and cumin, stirring well. Then add the diced tomatoes, tomato paste, chicken broth, beans, corn, shredded chicken, and any juices back into the pot. Bring chili to a gentle simmer, cooking for 20 minutes to allow the flavors to marry.
Serve chili in individual bowls, topped with cheese, tortilla strips, and avocado.