Mash garlic and salt into a fine paste using a mortar and pestle (or use the side of a chef's knife), and transfer to a bowl. Stir in cilantro, chili powder, oregano, lime zest, and olive oil. Cut 2 slashes in the skin of each fish fillet. Flip, and rub half the spice mixture onto fish; reserve remaining half.
Heat a grill pan over medium-high heat. Grill fish, skin side up first, flipping once, until cooked through, about 4 minutes per side.
Remove skin from fish, and discard. Flake fish into large pieces, discarding any bones. Combine fish with the remaining spice mixture and toss with 2 teaspoons lime juice.
Toss radishes, cabbage, and scallions in a bowl. Mash avocado in a bowl; stir in 1 tablespoon lime juice.
Using tongs, toast tortillas over an open flame of a gas stove, 5 to 10 seconds per side. (Or heat the tortillas in a skillet over high heat.)
Spread mashed avocado onto each tortilla. Top with fish, slaw, and cilantro leaves. Fold tortillas over filling, and serve with lime wedges.