Combine shrimp, olive oil, chili powder, cumin, and salt in a bowl. Place in the refrigerator to marinate for at least 15 minutes and up to 2 hours.
Prepare Pico De Gallo by mixing all the ingredients in a bowl. Cover and refrigerate until ready to use.
Prepare Guacamole by mixing all the ingredients in a bowl. Cover and refrigerate until ready to use.
Preheat your grill to medium high. Clean and season the grill grates. Grill the shrimp 2-3 minutes per side, until lightly charred and opaque.
Heat the refried beans and spread 2 T. beans on each tostada flat.
Spread the shredded romaine lettuce on top of the beans, followed by the Guacamole and the Pico De Gallo.
Top each tostada with 3-4 large grilled shrimp and sprinkle with grated Cotija cheese.
Garnish each tostada with a cilantro sprig, a lime wedge and enjoy.