Both have prep and both know about each others abilities.
Itachi will be healthy, Itachi will have all of his MS abilities.
Nagato is also healthy and can move around.
Round one: Itachi w/o Susanoo vs just Six Paths of Pain
Round two: Itachi w/o vs Nagato
Round three: Itachi vs Nagato and six paths
Battle takes place at the valley of the end
Battle theme is Pain's