The four presets are Story, Fair, Challenging and Duelist. They do not sit on a single ladder from soft to hard, because the hardest of them makes enemies easier to kill. It is worth reading what each one actually moves before you commit a thirty day run to it.
The four presets
Story makes enemies weaker and slower, and gives Coen more stamina. Fair is the default: every statistic on both sides sits where it was designed to sit. Challenging makes enemies more aggressive and harder hitting, and cuts Coen's stamina.
Duelist is the odd one. Enemies there have less health than on Fair, but they are far more aggressive, they hit much harder, and Coen has much less stamina to answer with.
Duelist shortens the fight in both directions
Less enemy health and much more enemy pressure is not a softer setting. It is a shorter one. Fights end quickly whoever wins them, and a single mistimed block can end one, because your stamina pool cannot absorb a second error.
If you enjoy the defensive game, that trade is the most interesting one on offer. If you want room to recover, Challenging keeps the pressure and gives you the stamina to survive being wrong.
The indicators are the real difficulty setting
On Story, Fair and Challenging the game tells you which direction to block, and it shows you when the perfect block window opens. Duelist removes both.
So the step up to Duelist is not a numbers change you can out-level. It is a reading test: you have to see the direction in the animation instead of in the interface. Play a few fights on Challenging with the indicators deliberately ignored before you decide.
One quiet mercy on Story
On Story you can drink blood directly from your inventory at any time. Every other preset makes you feed with Voracious Bite or reach a blood vial you put in a quickslot beforehand. For a first run that is a larger convenience than the combat changes.