Combine first 8 ingredients together in a bowl. Mix well. Add shrimp and toss to coat. Cover and refrigerate for 3 hours.
When ready to cook, preheat oven to 200 degrees to warm up the tostada shells. Cook 5 minutes or just until warmed.
While tostada shells are warming, heat refried beans in a pan over medium heat until warmed through.
At the same time cook prawns over medium high heat until cooked through, 3- 5 minutes.
To assemble tostadas, spread some refried beans on top of a tostada shell, spread guacamole on top, top with handful of lettuce and 5 - 6 prawns. Sprinkle with cojita cheese. Squeeze lime over the top. Repeat with remaining ingredients. Serve immediately.