This unusual pairing came to me earlier and I couldn't help but think... who would win in a deathmatch between these two wizened shinobi? Could Onoki's mastery of Dust Release and years of kage experience win him a battle against Danzo and his Izanagi?
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.
@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.
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.
Danzo should win with Koto otherwise Onoki can just Dust release anything Danzo has. Danzo is on a timer with Izanagi and Onoki can just sit in the sky make his earth clones and pressure Danzo. Onoki can make himself faster than Danzo by making himself lighter and heavier to withstand Baku. The only thing that's a concern other than what I've stated is if Danzo's wind jutsu can cut Onoki's earth wall or other earth jutsu but I'm not sure how the elements stack up against each other.