First or second time? In general bosses are bad at attacking right behind them and being able to outrange him with a spear can save you from his stupidly fast reaction attacks. A good shard (such as the chainsaw sword) can do a lot of damage quickly if you find a good place to use it. There's also nothing stopping you from cheesing fights with lots of healing items.I'm running into a serious wall with Katana Boss Guy (Kangetsu?). Is there a trick to beating him, or do you just have to be better at the game than I am at the moment?
For the first time I fought him I just reflexively jumped whenever he was close. He has an attack that punishes that but he doesn't use it much. His jump attack has a lot of start up time and takes a while before it can hurt you, making it an excellent time to dump damage onto him.
For the second time I did much the same but I had a lot more powerful ranged attacks and so I didn't need to pay as much attention to what attack he was doing. He can still do a lot of damage quickly, though.