Doom Blast, Decay damage is quite impressive

Nope. That is not a one-shot.
The damage actually takes 3 frames (at 60fps recording mode) to kill 5848 hp 75% chaos res character using Sould of Arakaali, Soul of Shakari.

And this is just lvl 19 Doom Blast with lvl 29 Decay at 477% damage from tree and gear. (and the blast itself has been blocked, you can see reckoning animation)

How can one play this game on arena as non-CI?
By removing curse immunity from the non-standard arena you have "turned off" any build which is unable to achieve chaos DoT immunity.

Someone could say "but this is not a oneshot, just use your flask idiot!"
Please remember about human reaction time, 100-150-250ms to hit the flask depending on age and training, 34ms server tick rate, 10-150ms client-server latency. Anything that happens faster than 0.3 sec is "too much"