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.
Pick up resilience for heavy blade, or it'll slow you for 3 seconds. You could also pick up someone like Atlas who can catch the heavy blade, someone like Zhin/Koga who can cleanse the slow, or someone like totemic ward Grohk to prevent the slow.
Tiberius deals 650 damage every .7 seconds, and .49 seconds during combat trance. That's a burst of 1300 every second for 4 seconds. Your best bet is to use someone like Lian to outburst him, or someone who can avoid his burst.
Cassie deals an additional 8% of a target's full hp with Big Game. If Raum has his full soul armor at 8k health, she does about 1320 a weapon shot for 4 seconds. That's 5 weapon shots, which is 6600 damage total, assuming she hits all of her shots.