In a medium bowl, combine the chili powder, cumin, oregano, garlic powder, onion powder, salt, pepper and olive oil. Toss shrimp in spice mixture and set aside.
Combine the avocados, cilantro, garlic, lime juice, salt and pepper for the guacamole in a medium bowl and set aside.
Combine the tomatoes, garlic, jalapeno, red onion and cilantro for the pico de gallo in a medium bowl and set aside.
Preheat a grill pan or skillet over medium heat. Spray with cooking spray.
To serve, divide the tostada shells between plates, top with guacamole, shrimp, and pico de gallo.