In a large bowl, whisk together the flour, baking powder, garlic powder, paprika, salt, and black pepper. Gradually add the beer, stirring until the batter is smooth and well combined. Let the batter rest for about 15 minutes.
While the batter rests, heat about 1 inch of vegetable oil in a large skillet over medium-high heat until it reaches 350°F.
Pat the fish fillets dry with paper towels and cut them into smaller pieces if desired. Dip each piece into the batter, allowing any excess to drip off.
Carefully place the battered fish into the hot oil, frying in batches to avoid overcrowding. Cook for about 3-4 minutes on each side, or until golden brown and crispy. Remove the fish with a slotted spoon and drain on paper towels.
Warm the tortillas in a dry skillet or microwave. Assemble the tacos by placing a few pieces of fried fish on each tortilla, then topping with shredded cabbage, diced tomatoes, cilantro, and avocado slices.
Serve immediately with lime wedges and hot sauce on the side.