In a bowl, combine the shrimp, olive oil, chili powder, garlic powder, cumin, salt, and pepper. Toss to coat the shrimp evenly. Let marinate for 15-20 minutes.
Preheat the grill or a grill pan over medium-high heat. Thread the shrimp onto skewers for easy grilling.
Grill the shrimp for 2-3 minutes on each side until they are pink and opaque. Remove from heat.
In a separate bowl, prepare the pico de gallo by mixing the diced tomatoes, red onion, jalapeño, cilantro, and lime juice. Season with salt to taste.
To make the guacamole, combine the mashed avocado, lime juice, and a pinch of salt in a bowl. Mix until smooth.
To assemble, spread a layer of guacamole on each tostada shell, top with shredded lettuce, grilled shrimp, and a generous spoonful of pico de gallo.
Serve immediately.