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.
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.
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 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.