These were the combos I used throughout the cups to beat all the MK8D Time Trial Ghosts:
1. Morton, Circuit Special (Frame), Cyber Slick (Tires), Cloud Glider (Glider)
2. Wario, Biddybuggy (Frame), Leaf Tires (Tires), Cloud Glider (Glider)
3. Donkey Kong, W 25 Silver Arrow (Frame), GLA Tires (Tires), Cloud Glider (Glider)
For the majority of the races I used the Morton or Donkey Kong combos. The only time I ever used the Warrior combo was for Dolphin Shoals, Warrior Stadium, and Neo Bowser City.
What combos worked well for you guys?