People tend to forget Kabuto was an eliet jounin level ninja even in part1. So it isn't surprising a rusty Tsunade ( who couldn't use her full power due to her fear of blood ) had hard time defeating him.
When you look at the manga you realise the kabuto fight wasnt as much of a beatdown as people would lead it on to be. Most of that comes from the anime I believe.
All that happens is he took the soldier pills and starts fighting with her she attempts to attack him but he gets her with chakra scapel (literally that whole thing happened in like 3-4 pages in the chapter).
Then she gets him with BPD.
Then she punches him and heals herself. He figures out BPD and is ready to exploit her blood fear and jiraya comes.
She pushes jiraya out of the way and was going to knock kabutou out point blank and he cut himself.
Fight over. cuz at that point even naruto could have knocked tsunade over how she was crippled with fear (hardly impressive) he knew he had to use her blood fear to hope to win.
As you can see the most he did in that fight for the most part was chakra scapel after he did thaat to me he got an equal share of spanking. Also like u said he was considered to be near or at pt 1 kakashi level soz rusty tsunade should have a bit of a bad time.
