Nagato wins with 0 difficulty. Skilled Rinnegan users can use multiple paths at once without time intervals, this is shown by Nagato (as well as reconfirmed by Kurama when he tells Naruto he is lucky Sasuke has not learned to do so yet).
Nagato can absorb any technique Oonoki uses, except MAYBE earth techniques. (can he absorb moving earth if its a jutsu using chakra? logic says no, but he absorbed toad oil, and fire, and technically, those are physical things too).
even if he could not, Oonoki has no answer for Asura path or Human path. Matter of fact, Bansho tenin him into the ground and his spine breaks and he is paralyzed.
0 Difficulty.