If cooking outside on the grill, soak skewers at least 30 minutes.
Place the pulp from the avocados in a medium bowl and slightly mash with a fork or a potato masher leaving some large chunks. Add tomato, lime juice, salt, pepper, cilantro, red onion, garlic and mix thoroughly.
Season shrimp with salt, then mix in crushed garlic and 2 tbsp salsa verde. Thread the shrimp onto 4 skewers.
Light the grill or indoor grill pan on medium flame, when hot spray the grates with oil and grill the shrimp about 1 to 2 minutes on each side. Set aside.
Place a tostada on each plate, top each with 1/4 cup refried beans spread evenly, 1/4 cup lettuce and guacamole.
Top each with a shrimp skewer on top and serve with extra salsa verde and cheese if using.