Mash the avocados with the rest of the guacamole ingredients or whizz everything together in a blender or food processor. Cover tightly with plastic and set aside.
Toss the shrimp in 2 teaspoons chili powder, 1 teaspoon garlic powder, lime juice, 2 tablespoons olive oil, and salt and pepper to taste. Spear the shrimp onto prepared skewers and set aside.
Take the mangoes and cut the flesh from the pit on either side into 2 large flat slices, leaving the skin on. Use your knife to crisscross the mango in a grid pattern. Brush with olive oil.
Heat the grill until it’s searing hot. Grill the shrimp (without moving them) for 3 minutes on one side. Flip and grill for another minute on the other side. At the same time, grill the mangoes for about 1-2 minutes per side to caramelize them. Transfer everything to a plate.
To assemble the tacos, warm the tortillas or place them on the grill to toast. Spread some guacamole on each tortilla, and top with the shrimp, mango, chopped tomato, and cilantro. Serve with a squeeze of lime.