Kakashi wins mid difficulty. Tsunade would have a lot of troubles trying to overwhelm Kakashi because she doesn't possess powerful long range techniques. She could summon Katsuyu which Kakashi can't kill unless he wastes a large kamui but all it takes for Kakashi to win is a headshot with kamui. Kakashi managed to kamui KM Naruto's rasengan so he should be able to snipe Tsunade's head. He does know about her regeneration, or at least he should so he is bound to resort to it knowing Tsunade can't be put down through regular means.
Without kamui, it would be a closer fight but I see Tsunade winning. Lightning probably won't paralyse Tsunade since she also has an affinity for raiton and Sasuke showed us that raiton doesn't paralyse other raiton users when he got hit by Darui's lightning transmission.
Kakashi won't be able to behead her with raiton variants since he has to get close to her in order to do so and if he does come close, Tsunade would simply punch him away. She'll outlast Kakashi or manage to destabilize his footing and then kill him. She also has the option of spreading Katsuyu minis to surround Kakashi since Kakashi doesn't have any wide area of effect attacks, he'll eventually be overwhelmed and this would give Tsunade an opportunity to punch him.