1 on 1 battle, no interference form Raikage, Konan or anybody else.
Pain of course has all six paths and can use all jutsu he did against Naruto
Killer Bee will not hesitate to use stage 3 transformation
They are bloodlusted and are trying to take each other out.
If bee manages to take out the six paths, he does not have to kill Nagato to make it count as a win.
No particular battlefield just a fair one.