The Hokages are kinda stacked. One thing the Hokages already have over the kages are numbers. All the hokage's minus tsunade have SHADOW CLONES, which already put the kages at a disadvantage in terms of numbers. (Sand and Rock clones arnt as efficient as shadow clones).
Now you have Tobirama and Minato, who can place there seals on the party, which will allow tobirama to teleport them out of harms way if need be.
Follow that up with the fact that tsunade can instantly heal anyones chakra reserves or wounds due to tobirama's ability to teleport her around the field with his markings (Or minato), which means Spamming Chakra taxing jutsu would be a problem.
Finally you have a Shinobi who can not only make 5 shadow clones of himself, but each of the clones has every nature element.
The Hokage's are stacked, if they work together.
Even with Onoki having Flight, if minato gives hiruzen a marked kunai and hiruzen makes 1000 (Shuriken Shadow Clone) of them all while lauching it at onoki, they wont be able to avoid them all which would allow minato to attack the from a kunai that could possible exist in his blind spot.
The hokages take this with Mid-High Difficulty. With Gaara being the most difficult to defeat