Prepare a large baking sheet with a wire rack on top and set aside.
Heat the oil: pour approximately 1/2 an inch of oil into a dutch oven or heavy bottomed pot. Heat over high heat, oil will be approximately 375 degrees F. Since it’s shallow it’s hard to take the temperature, but the oil will appear to shimmer when it’s hot enough.
Prepare the batter: add the cornstarch to a small bowl. In a medium size bowl add the flour, baking powder, salt and chili powder. Pour in the cold beer and stir until just combined.
Prepare the fish: Cut the fish into strips, 1/2 inch by 2-3 inches. Pat the mahi mahi dry and dip into the cornstarch shaking off the excess.
Working in batches, dip the fish into the beer batter, scraping off the extra.
Carefully lower the fish into the hot oil and let fry for approximately 2 minutes, then turn and continue frying the other side an additional 1-2 minutes
Remove the fish from the oil and place on the wire rack.
Continue working until all the fish is fried.
Assemble the tacos: heat the corn tortillas on the stovetop until lightly charred on the edges or just until warm and pliable.
Layer the tortillas with coleslaw or cabbage, fried fish, pico de Gallo, crumbled queso fresco, and a drizzle of fish taco sauce.