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.
Serve.