VS
Context:
CAS is a fully conceptual entity capable of both holding Limbo (an outerversal realm - it exists "above" the main DC multiverse) on the tip of his finger and sensing the breath of the comic reader.
He is a literal plot device created to defend the DC macro-structure against omni-versal threats.
Itachi has a hard counter though that makes this fight interesting. The legendary Yata Mirror.
Distance: Nil or Earth Prime (CAS is much bigger than the DC multiverse so assume Itachi is sized up)
Mindset: Bloodlusted
Starting Distance: 20 meters
Who wins and how hard is the fight? My take.
Itachi stomps no or low difficulty