In a mixing bowl, combine olive oil, lime juice, chili powder, ground cumin, garlic powder, salt, and black pepper. Whisk together until well blended.
Add the chicken breasts to the marinade, making sure they are evenly coated. Cover and let marinate for at least 15 minutes (or up to 2 hours in the refrigerator for more flavor).
Preheat the grill or grill pan over medium-high heat.
Remove the chicken from the marinade and discard the marinade. Place the chicken on the grill. Cook for 6-7 minutes on each side, or until the chicken is cooked through and has an internal temperature of 165°F (75°C).
Once cooked, remove the chicken from the grill and let it rest for a few minutes before slicing into strips.
While the chicken rests, warm the tortillas on the grill for about 30 seconds on each side until soft and pliable.
To assemble the tacos, place a few strips of grilled chicken in the center of each tortilla. Top with shredded lettuce, diced tomatoes, onions, chopped cilantro, and avocado slices.
Serve with lime wedges on the side for extra flavor.