Heat oil in a large skillet over medium-high heat. Add corn and sauté 5 minutes or until corn is roasted. Stir in black beans, tomatoes, lime juice, salt, and, if desired, red chili flakes. Reduce heat to medium-low and cook 3 minutes or until the ingredients are warm.
Spread guacamole evenly over the bottom of 4 tostada shells. Top each shell evenly with sliced grilled chicken, corn and black bean mixture and fresh cilantro.