In a large bowl, combine the chicken thighs with the olive oil, smoked paprika, chili powder, cumin, garlic powder, oregano, salt, and pepper. Massage the spices into the meat until fully coated. Let marinate for at least 30 minutes.

While the chicken marinates, prep your toppings: finely dice the red onion and tomatoes, chop the cilantro, and crumble the cotija. In a separate small bowl, mash the avocados, then stir in the lime juice and salt to create a simple guacamole.

Preheat a grill or cast-iron skillet over medium-high heat. Cook the chicken for 5-7 minutes per side, until a crust forms and the internal temperature reaches 165°F (74°C). Transfer to a cutting board and let it rest for 5 minutes.

Chop the rested chicken into small, bite-sized pieces. Warm the corn tortillas over a gas flame or in a hot, dry skillet for 15-30 seconds per side until pliable and lightly charred.

Layer the chopped chicken onto each warm tortilla. Top generously with diced red onion, tomatoes, cilantro, and cotija cheese. Finish with a dollop of guacamole and serve immediately.