How is itachi beating pain other than potential tsukiyomi GG? Pain can simply spam summonings which will overwhelm itachi at one point. Even healthy itachi has limit on amaterasu spam and susanoo.
Let me answer that for you .
First off amatarasu literally destroyed all pains summonings an so did a kunai .
Secondly itachi proved he can destroy multiple rennigan an its visual link in an instant with just kunais an base sharingan something that costs no energy so the question you should be asking is how pain beating itachi when he gets his rennigans destroyed which literally happens in the anime .
3rdly regular genjutsu worked against pain an ma an pa literally hated using it an complained about how hard it was to do meaning they didn't master the jutsu perfectly yet it was very effective against pain . Itachi is literally the god of genjutsu he can literally put you under genjutsu without using his eyes . He can literally use crow clone genjutsu an countless others an is a master of it he doesn't even need tsukuyomi to beat pain as regular genjutsu works on him proven by jiraiya , MA an PA.
Pain literally has no counter for touska blade an touska blade can be manifested just like a rib cage or rib, or shell,or half susano , or a skeleton frame ect so can an arm or touska blade meaning you have no idea when touska blade can come as it can literally be used as a sneak attack as it can be manifested alone without the susano it self just like the rib cage ect.
Dying itachi speed is faster than pain why because he died after his body an stats was nerfed by his illness an reanimation revives you with the body an stats you had just before you died hence why hiruzen was old an Minato, tobirama , hashirama had prime bodies as they died before getting nerfed by old age. So itachi stats in reanimation is his dying stats at 90% as he wasn't revived at 100% so this is not even his healthy stats nor was he reanimated at 100%
So far I've listed multiple ways itachi beats pain all of which he has no counter to as its proven at some point in the anime .