Combine soy sauce, agave, vinegar, garlic powder, ginger and oil in a container with a tight lid. Shake to mix well. Make light diagonal slashes on each side of the flank steak in a diamond pattern. Place meat in a small pan and pour marinade over it. Cover and place in the refrigerator for at least 4 hours. Prepare the grill for cooking on medium-high heat. Cook the steak about 6 to 8 minutes on each side, remove from grill and let steak rest for 5 minutes.