If Nagato still has BT, then he should win low-mid difficulty.
There's nothing stopping him from using BT/Asura cannon on Jiraiya from the get-go. Once Jiraiya falls, Nagato uses Rain Tiger to notify him of any Zetsu shenanigans, and his bird summoning to keep outside the range of Tsunade and Oro, who have no real ranged attacks to speak of. From that point on, Nagato can pick off the remaining Sannin with BT/soul-rip, using Asura arms/cannons to deal with any close-range complications that might arise from this strategy.