The Life&Death Charm. If you craft your own weapon with this charm on it, your vitality will regenerate automatically. This means you no longer have to buy healing items, freeing inventory space for other handy items, such as the strength amplifying wine. It also allows you to fast travel without any drawbacks, as fast travel normally costs you vitality, which your weapon will regenerate for you.
Combining the above with making your weapon Unbreakable will make the game very easy, even on the highest difficulty. If your weapon possesses the Life&Death Charm, you won't have to worry about healing or repairs ever again, allowing you infinite staying power in battles.
Creating your own combat style can become this. Pick a style you are comfortable with, throw every useless technique out, replace these with more powerful ones, throw all passives in and you have a style with virtually no weaknesses that allows you to bulldoze over all your enemies.
Good Bad Bugs: One of the moves from the Gan Swordfighting style, Gazan Shindakyaku, has an unintended glitch that causes it to register a second kill against your victim after you connect the move with your sword if you sufficiently wound your victim enough to prepare for the finishing kill or your sword has a high enough strength. This can be quite useful for leveling up your sword when meeting with the local blacksmith.
Nightmare Fuel: In one of the bad endings, your character and others are boiled alive! Since this is most likely the first ending people get, we don't know what happened to them afterwards. Then we found out what happen to those characters after avoiding the ending the next time. It was implied that they were eaten by the minister and his daughters!
Player Punch: Some of the bad endings qualify. Especially the ending in which your character gets boiled alive. Said ending has a Hope Spot, in which your character still hangs above the boiling cauldron. However, Kinugawa just gloats evilly and sadistically throws your character in as well. Since this is likely the first ending people get, it will probably motivate them enough to see Kinugawa dead.
Both fights against the Kinugawa sisters in the hidden story path can feel like this, but the second one really deserves this title. As you are captured by the Big Bad and given to his daughters as their new plaything, you need to fight all three of them, at the same time, without your weapons and items. If you haven't trained yourself in any of the unarmed martial arts schools, this fight will be next to impossible.
The two bosses from the Shinsengumi DLC, Kondo and Hijikata. Both have highly powerful swords and they can easily kill you even on Easy difficulty (hence, never take them on when playing in Hard or Harakiri mode). The fight with Kondo is particularly difficult as he cannot be harmed in regular mode unless you use reverse mode to deplete his vitality completely to which he then becomes vulnerable on regular mode. Hijikata's fight is even worse in that even if you completely drain his health, he always has three healing medicines to fully recover his health and vitality points, forcing you to repeat the process and avoid getting killed by his dangerously powerful attacks. You must have a particularly good fighting style, a good amount of healing items such as Onigiri, Red Pepper, or Steamed Cake, and a sufficiently leveled up sword to defeat these two bosses if you want to get their special outfits and swords, as it's very well worth it after finally defeating them.
That One Level: Related to the That One Boss entry; The events leading up to said fight include fighting a bunch of Demonscales and surviving all three torture minigames. The last part is especially frustrating, as no amount of preparation and upgrading your sword will make these minigames easier. To make matters worse, if you lose the fight against the sisters, you will need to do everything all over again, as there are no checkpoints between all these events.
