If Neji engages into Hand to hand Taijutsu combat he is going to lose right away. All Tsunade has to do is use this
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.
If he keeps his distance he is unable to attack due to his lack of an
effective long range Jutsu.
All Tsunade has to do in that case is land a massive punch to the ground, and Neji will be off balance. He cannot spin in those circumstances so he will get killed by Tsunades follow up punch.
Even if Neji somehow survives he doesn't have the stamina of a Senju to keep up with Tsunade. Since all he can do is defend against attack after an attack he will get worn out eventually. While Tsunade wouldn't even have to use half her power in her base form. (No Summon no Seal ). And don't forget the battle experience, Neji has maybe 2-3 years of actual battle experience. (Chunnin+) While Tsunade has decades of Battle experience.
Tsunade takes this very low diff. Neji has slim to none chance at even landing a hit.