Preheat grill or stovetop grill pan over medium-high heat.
In a small bowl, mix together olive oil, cumin, chili powder, garlic powder, salt, and black pepper to create a marinade.
Rub the marinade all over the flank steak, making sure to coat both sides.
Grill the steak for 4-5 minutes per side, or until desired level of doneness is reached.
Remove the steak from the grill and let it rest for 5 minutes before slicing it thinly against the grain.
Warm the corn tortillas on the grill or stovetop until they are pliable.
Assemble the tacos by placing a few slices of steak on each tortilla and topping with salsa, guacamole, chopped cilantro, diced red onion, and queso fresco.
Serve immediately and enjoy!