Depends on how you introduce difficulty. Just making enemies hit harder and take a ton of hits is not a good way to increase difficulty. Souls games largely handle difficulty with enemy placement and hard to predict enemy moves. NG+ modes largely just compensate your weapon level and stats.
I think Nioh has enough enemies that can take lots of hits to take down but most enemies have a very limited moveset that doesn't cover a lot of bases as you can usually just dodge behind them.
That was the case for Souls as well, most enemies can be circumvented by dodging through their attacks and hitting them in the back. Souls have a lot of 'variety' but most of those don't actually have difficult moves.
So anyway back to Nioh, Youkai are easy to make interesting since you can get them to do whatever and make them crazy looking. So for human enemies I think the way to go is to make them progressively more like the player characters. For example:
1) Bandits - only uses mid stances
2) Bandit Leader - uses all stances
3) Samurai - uses all stances, parries
4) Samurai Elite - uses all three stances, parries and some skills
5) Samurai Commander - uses Ki pulse, all stances, all skills
This can extend to Ninja, Onmyouji, or hell even warrior monks