I see both side can one shot this, if pain had knowledge he would go for a blitz right away.
Onoki would probably be in air, so Pain's only option would be Bansho which may or may not work on Onoki.
Onoki could oppose it with his flight, or he could make himself superheavy to avoid it.
Then Onoki process to Jinton the entire area only to find out pain can absorb it.
If Onoki have no knowledge the fight would end here as he won't know how to counter Preta's ablilty.
Even he he had knowledge it would be really hard to take out Preta with other pains defending him.
If Onoki can take out Preta he pretty much have this fight in a bag.
Again pain could always go straight for CST and Onoki ain't gonna do nothing but die.
Pain 8/10 high difficulty.