FOR COLESLAW: Toss all ingredients in medium bowl to blend. Season slaw to taste with salt and pepper. (Can be made 8 hours ahead. Cover and chill.).
FOR SAUCE: Blend all ingredients in mini processor. Add water by teaspoonfuls to thin if desired. Season sauce to taste with salt. (Can be made 8 hours ahead. Cover and chill.).
FOR MAHI-MAHI: Combine first 3 ingredients in large bowl; gradually whisk in beer. Place fish in large strainer. Spoon additional flour over. Toss fish in strainer to coat. Shake well, releasing excess flour. Mix fish into beer batter. (Can be prepared 1 hour ahead. Let stand at room temperature.).
Pour enough vegetable oil into heavy medium saucepan to reach depth of 1/2 inch. Attach deep-fry thermometer to side of pan and heat oil to 375°F Working in batches, lift fish from batter and drop into hot oil. Fry until crisp and golden, about 1 minute per side. Transfer fish to paper towels to drain.
Cook tortillas in large skillet over medium-high heat until warm and pliable, about 20 seconds per side. Stack 2 tortillas for each taco. Using tongs, place some slaw in center of each tortilla stack. Top with sauce, cilantro sprig, and fish. Fold edges of tortillas over filling. Repeat with remaining ingredients. Place 2 tacos on each of 4 plates. Garnish with lime wedges; serve.