In a mixing bowl, combine olive oil, lime juice, chili powder, cumin, garlic powder, salt, and black pepper. Mix well to create a marinade.
Add the chicken thighs to the marinade, ensuring they are well coated. Let the chicken marinate for at least 10 minutes.
Preheat your grill or skillet over medium-high heat.
Once hot, place the marinated chicken thighs on the grill or skillet. Cook for about 6-7 minutes per side, or until the internal temperature reaches 165°F (75°C).
Remove the chicken from the heat and let it rest for a few minutes. Then, slice it into bite-sized pieces.
Warm the corn tortillas on the grill or in a skillet for about 30 seconds on each side.
Assemble the tacos by placing sliced chicken on each tortilla. Top with diced onions, chopped cilantro, diced tomatoes, and avocado slices.
Serve with lime wedges on the side for extra freshness.