In a large freezer bag or Ziploc bag, combine margarita mix, lime juice, olive oil, salt, pepper and garlic powder; stir until well combined and add chicken.
Close bag and refrigerate for at least 4 hours.
Coat grill pan with cooking spray and set over medium-high heat.
Remove chicken from ziploc bag and discard sauce.
Add chicken to grill pan and cook for 6 minutes; flip and continue to cook for an additional 5 to 8 minutes, or until done.
In the meantime, prepare the salad.
In a large salad bowl, layer chopped iceberg lettuce.
Add corn, black beans, tomatoes, red onion, and avocado slices.
When chicken is done, slice into strips and add on top of salad.