In a medium mixing bowl, whisk together lime juice, olive oil, chili powder, garlic powder, cumin, salt, and black pepper.
Place the chicken breasts in a large resealable bag or a shallow dish and pour the marinade over them. Seal the bag (or cover the dish) and refrigerate for 15-30 minutes.
While the chicken is marinating, prepare the avocado salsa. In a mixing bowl, combine the diced avocados, red onion, tomato, and cilantro. Drizzle with lime juice and season with salt and pepper. Mix gently to combine.
Preheat the grill or grill pan over medium-high heat. Lightly oil the grill grates to prevent sticking.
Remove the chicken from the marinade and discard the marinade. Grill the chicken for 6-7 minutes on each side or until fully cooked (internal temperature should reach 165°F or 75°C).
Once cooked, remove the chicken from the grill and allow it to rest for a few minutes before slicing.
Serve the sliced chicken topped with a generous scoop of avocado salsa.