Preheat the grill to medium-high heat.
In a bowl, combine the shrimp, olive oil, chili powder, garlic powder, cumin, salt, and pepper. Toss until the shrimp are evenly coated.
Thread the shrimp onto skewers for easy grilling. If using wooden skewers, soak them in water for 30 minutes beforehand to prevent burning.
Grill the shrimp for about 2-3 minutes on each side, or until they are pink and opaque. Remove from the grill and let cool slightly.
To assemble the tostadas, spread a layer of guacamole on each tostada shell.
Top with grilled shrimp, pico de gallo, and shredded lettuce.
Serve with lime wedges on the side for squeezing over the tostadas.