Yomi Numa is associated with Konoha through Jiraiya.
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I have no reason to believe Hiruzen isn't aware of the technique. Just by looking at the 5 techniques Tobi's several thousand hands was about to perform, he replicated all of them. Jiraiya is Hiruzen's student, as well, just increasing the likelyhood of him knowing it, if that statement by Orochimaru of "you know and can use every technique in Konoha" didn't cement that fact enough.
And there's numerous ways the battle could unfold.
Hiruzen and Danzo summon immediately to start. Enma creates several clones(Hiruzen and Danzo could as well, just to add to the difficulty for the paths) as they, Hiruzen and Danzo rush the paths. Deva, predictably, uses shinra tensei to end their assault, which Hiruzen nullifies with Yomi Numa. With the 6 second countdown activated, they continue their assault(Enma creating more clones if need be, incase the shinra tensei did manage to do enough to some of the clones out), as all of the clones deals with Asura, Human, Animal and Preta(Baku assists if Animal summons), and either Hiruzen or Danzo ends Deva, the most effective method possibly being Danzo ending him via any sort of futon, such as his futon blade, Shinkujin. Deva can't take Hiruzen, Enma in staff form and Danzo all at the same time in CQC. The clones might not last too long overall but the sheer number of them should buy enough time for Hiruzen and Danzo to pull off the Deva assassination.
Baku can then suck in Deva's body to prevent restoration by Naraka, but if he's too busy dealing with Animal Path's summons, Hiruzen and Danzo just need to play keep away with Deva's body. This can be done with a clone taking Deva's body away from the battle field as Hiruzen and Danzo continue their assault.
As Baku continues to deal with the Animal Path summons as best he can(which admittedly may not be terribly long, but so long as he can buy a little bit of time he's done his part), Hiruzen, Enma and Danzo help the remaining clones that still exist and focus on taking out Asura first. Hiruzen is best equipped for this task given Enma can smash Asura in staff form, and can also defend Hiruzen from Asura's weapons.
Human is an issue with sneaking up on Hiruzen and Danzo after he defeats the clones assigned to him, but Danzo's izanagi and Enma for Hiruzen can save them in this regard. Beyond the ability to sneak up on them and remove their souls, he simply doesn't pose much of a threat to these two skill wise.
The only remaining path, who skill wise poses a threat, is Animal, but again given their full intel here, they're going to simply avoid confrontation with the summons and go after the path itself.
They'll know Preta's only use is absorbing ninjutsu, and Human's only real threat to them requires him performing a sneak attack, so the fight becomes pretty easy, really from the point that they eliminate Deva, so long as Naraka is kept away from his body.
For proof that Enma can create enough clones to cause issues here(and prevent Hiruzen and Danzo from having to create too many/waste too much chakra), he used numerous clones to create the cage that protected Hiruzen vs the mokuton.
Credit to Blaze Release for the edit on this image, pointing out the number of Enma clones:
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That's 18 clones there, and Enma showed no signs of exhaust from that, so no reason to believe he can't create even more if need be.
Their plan here, regardless of how you want to design it(and I could develop other strategies for it if need be), is distract the other 5 and get rid of Deva ASAP. And with the amount of clones Enma can create(plus the clones Hiruzen and Danzo could create if need be), and Baku's assistance with Animal's summons, distraction is no issue.
