Danzo would win, he's too fast for Onoki, his izanagi allows him to survive particle style, he's stronger physically, he's smarter, he has much more chakra and is in better physical condition.
Senior vs. Senior.
Danzo has several wind techniques that can make Onoki's flight a problem. Danzo has several lives and can reappear anywhere. He has a summoning that can easily suck Onoki up.
Onoki can use rock clones. Onoki can use other Earth Release jutsus, both for offense and defense, but Danzo's arsenal of jutsus will be able to handle it. Onoki has Dust Release, which seems about the most effective, but Onoki will need distance and be able to find some way to distract Danzo to land it as Danzo is fast.
I'm putting my money on Danzo with mid difficulty.
^^Ohnoki can just cancel danzo's summoning using fire release the same way sasuke did.
@Kratos You do realize that the only reason Sasuke didn't get sucked in right away and had time to use the Fireball Jutsu was because of Susanoo. Onoki doesn't have such a thing to defend against, especially being in the mid air. Also, we don't know any fire jutsus Onoki has so him having Fire Release is irrelevant.
Tasuxeda Imagine That Its Post-Kage Summit, And Danzo Has Already Used Koto, But Before His Fight With Sasuke.
OP didn't specify that the fight takes place Post-Kage Summit, but if it was then Danzo would only have a chance if he can gets into close combat with Onoki otherwise Onoki would likely fly about out of Danzo's reach dodging his attacks and use dust release to force Danzo to use up his eleven minutes of Izanagi based on Onoki's performance during the war I think he could out last Danzo.
Izanagi is the deciding factor. With it, Danzo wins. Without it, he loses.
onoki, because i hate danzo, also onoki can just stay in the air until the izanagi wears off completely, use the weaker jutsu until izanagi is completely gone and then particle style