Deva Path is the biggest threat here.. he's pretty much the only one who can secure the victory for the 6POP so with manga knowledge on them Tsunade should let the Kages know to take him out first.. anyways I see some of you saying that he'll use CST right off the bat. I believe that's a bad move seeing as how they'll tank most of the damage by Tsunade using Katsuyu and then Gaara using a big wall of sand to protect them as well. After using it he'll lose his powers temporarily leaving him vulnerable to attack.. the other 5 paths won't be able to hold off 5 kage level shinobi individually before Deva Path recuperates his powers. The only one that's going to put up a fight is animal path but with Tsunade using Katsuyu the acid will kill Cerberus which is the biggest threat as a summon.
I believe the Kages win low-mid difficulty but then again someone might provide an argument for the 6POP which may change my mind.