Tiberius can be damage immune during the whirling blade part of his ult. Use someone like Io or Ash, people who can cc him off the map. He can't use another move until he lands in his whirling blade stance, so he'll fall to his death.
Tiberius has no damage fall off, meaning 650 damage every .7 seconds and .49 seconds no matter what range you're at. The best counter is someone like eminence Lian or a sniper who can deal more damage than Tiberius from afar.