Place the chicken breasts in a large pot with the chicken broth and bring to a gentle simmer. Cook for 15 minutes until the chicken is fully cooked through, then remove the chicken to a plate and shred it with two forks.
In the same pot, add the onion and garlic to the simmering broth. Cook for 3 minutes until the onion softens.
Stir in the cumin and chili powder, then add the diced tomatoes with their juices. Return the shredded chicken to the pot.
Add the corn and black beans. Season with salt and pepper. Simmer for 10 minutes to let the flavors blend.
While the soup simmers, spread the tortilla strips on a baking sheet and bake at 400°F for 8 minutes until crisp.
Ladle the soup into bowls and top with the baked tortilla strips, cilantro, lime wedges, and avocado slices.