If using wooden skewers, soak them in water for at least 30 minutes before skewering food.
Cut onions, tomatillos, and bell peppers into large, skewer-sized pieces. Assemble skewers, alternating 1 piece of each vegetable between 5 shrimps on each skewer. Arrange skewers in a large non-reactive baking dish and pour the marinade over. Submerge skewers in marinade, and marinate, covered, in the refrigerator, for 4 to 6 hours.
Preheat a grill to medium-high heat. Grill skewers for approximately 6 minutes, turning as needed. Remove from the grill and serve.
5 ounces chopped garlic
2 ounces garlic powder
1-ounce ground cumin
1-ounce freshly ground black pepper
2 ounces paprika
4 ounces pineapple juice
32 ounces vegetable oil
1 bunch cilantro, leaves chopped
Combine all ingredients together in a large bowl and mix well. Pour marinade over skewers as directed above.