Preheat oven to 400 degrees F. Lightly coat both sides of corn tortillas with coconut or olive oil using a brush. Bake tortillas for about 4 minutes on each side. (They will continue to crisp up as they cool.)
Toss shrimp with olive oil, chili powder, and ground cumin. In a medium skillet over medium-high heat, cook shrimp until shrimp are bright pink and opaque throughout. In a small saucepan, warm the beans over medium heat.
Assemble the tostadas by layering the beans, then the guacamole, and finally add the shrimp on top. Sprinkle with tomato and lime juice.