Whisk together lime juice, olive oil, garlic, chili powder, cumin, paprika, salt, pepper, and red pepper flakes in a small bowl.
Add shrimp to a bowl. Toss to coat and marinate for 15 minutes.
Preheat the grill to medium heat. Put shrimp on skewers and place on the grill. Grill on each side for about two minutes or until no longer pink.
Put avocado, Greek yogurt, garlic, lime juice, and cilantro in a food processor. Pulse until smooth. Add salt and pepper to taste.
Arrange the shrimp on top of a plate of mixed greens. Serve immediately with avocado-lime sauce on the side and slices of lime for decoration.