ive been seeing a whole lot of repeats but basically its like this , ( people double check the manga before you come up with assumptions on the souls, how long they last or how weak they are from the original , cause the 3rd hokage didnt mention anything on the souls that he was fighting if they were weaker then the originals which he wouldve made clear )
Sarutobi also said himself even if he killed Orochimaru the souls wouldnt die you have to kill them also ,
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SO basically what makes a person much stronger then 2 Hokage's and a Orochumaru is something called the Will of Fire , something you protect with your life and stop at nothing to ensure your obstacle ahead of you is completed whether it takes your own life or not .
thats most likely how 1st hokage beat madara
thats sarutobi stopped the 2 hokages and snakes tounges arms
and how a single man managed to put down a thing like the 9 tails 8 ( 4th hokage )
and many more
P.S .
even if itachi or nagato didnt have the eyes , ide guarentee itachi has equivalent shikamaru brain cells and nagato has every single elemental masters and shinra tensei and more