In a large bowl, combine the first 7 ingredients. Add the fish, season with salt and pepper and toss to coat. Let stand 15 minutes.
In a medium bowl, combine the onion, chile, lemon juice and garlic. Sprinkle with salt and let stand until juices form, about 10 minutes. Scoop the avocado into the bowl and mash with a fork. Stir in the corn, tomato and cilantro; season.
Heat a large nonstick skillet over medium-high. Cook the fish, working in batches if needed, until lightly browned and opaque in the center, 2 to 3 minutes per side.
Heat a small skillet over high. Add one tortilla and cook until blistered in spots, 20 to 30 seconds per side. Wrap in a kitchen towel to keep warm. Repeat with the remaining tortillas.
Fill the tortillas with the lettuce and fish; top with the guacamole.