The only logical way i can see Greenbull winning this is by using a sneaky hax of some sort like Caesar clown or taking them off guard. Otherwise there's just no way he would solo the alliance.
I think people must realize that the alliance in Wano is much stronger than Whitbeard forces on Marineford (an old and sick yonko whose strongest support was Marco). Saying Aramaki will solo the alliance fairly is like saying him alone could have put a stop to the Marineford war or that he can solo Whitebeard fleet plus an additional Yonko. This would make absolutely no sense, ain't no way this is happening
Oda will probably make him look good for a couple of chapter than he will be put in his place and retreat after maybe taking something or someone from them. This is not sabaody, pre timeskip or dressrosa, nobody will run away
Luffy, Kid, Law, Yamato, Zoro, Marco, Jimbei, Killer, Sanji and many more losing against Greenbull ? Hell no