Whisk together the paprika, ancho powder, brown sugar, cumin, salt and pepper in a small bowl.
Place the shrimp in a large bowl, add the spice rub and stir well to coat each shrimp.
Heat your grill to high. Heat 3 tablespoons of the oil on the grates of the grill over high heat in a large cast iron or saute pan; add half of the shrimp and half of the garlic and cook until just cooked through, 3 to 4 minutes. Stir in half of the green onions and transfer to a large platter or turn out onto brown paper bags. Wipe out of the pan with paper towels, and repeat with the remaining ingredients.