Bring a pot of water to a boil and poach the shrimp for 2-3 minutes until pink and opaque, then immediately plunge into an ice bath to stop the cooking.
Once cooled, peel and devein the shrimp, then chop them into bite-sized pieces and place in a glass mixing bowl.
Add the lime juice, extra virgin olive oil, chili powder, sea salt, and black pepper to the shrimp, tossing well to coat.
Finely dice the cucumber, tomato, and red onion, then fold them into the shrimp mixture along with the chopped cilantro.
Cover the bowl and refrigerate for at least 15 minutes to allow the flavors to meld and the shrimp to absorb the citrus.
While the ceviche chills, toast the corn tortilla in a dry skillet over medium heat until it becomes a crispy tostada.
Just before serving, gently fold in the diced avocado to maintain its creamy texture.
Spoon the ceviche onto the crispy tostada or serve in a bowl with the tostada on the side.