Tsunade low-mid difficulty. Darui's lightning attacks won't do much to Tsunade and she can easily regenerate from those attacks. I don't think Darui can paralyse Tsunade since apparently Tsunade is also lightning natured. In CQC, Darui has no chance and at long range, Tsunade can simply summon Katsuyu to continuously spit acid on Darui. One punch from Tsunade or some acid will kill Darui. On the other hand, Darui's close range sword attacks or long range lightning attacks will be useless unless he can decapitate her. Still the idea of someone on Darui's level decapitating Tsunade seems very very unlikely.
All in all, Darui seems to be on a huge disadvantage in this fight and he can't do much against Katsuyu either. Maybe I should reduce the difficulty to low difficulty when I actually come to think about it. Don't want to underrate Darui but he is horribly outclassed.
Also what is up with the Tsunade threads? Apparently she gets compared to EMS Madara who is far above her level and now she gets compared to Darui who is far underneath her. Threads these days...U_U