I'll make some general comments for the IY side:
-Naraku can hide his presence behind an invisible barrier.
-Naraku has tons of demons, enough to cover up the skies.
-Naraku's presence and miasma taints the weaker spirits around him.
-Kanna is dangerous here. No one in Bleach can detect her, because she has no spirit presence, so she sneaks up behind and capture their spirits. She'll have to unload them somewhere though so her mirror doesn't break.
-Mouryomaru is almost as dangerous as Naraku. Both of them can collect powerful Shinigami and Hollow abilities for themselves.
-Byakuya can be dangerous too because he is an illusion user like Aizen. Bleach verse doesn't seem to have much defense against illusions.
-His psionic spiderweb converts lots of Shinigami and Hollows to his side.
Naraku and his crew would probably start by going into hiding and building up their power.
All he really needs to solo is his shield and miasma, even without his shield it's doubtful they're cutting his hide apart. And without that still his regen keeps any of them from landing the finisher.