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 minutes.
Preheat a grill or grill pan over medium-high heat. 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 guacamole by mixing the mashed avocado, lime juice, and a pinch of salt.
For the pico de gallo, combine the diced tomatoes, red onion, jalapeño, cilantro, and salt in another bowl. Mix well.
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 and enjoy!