Trek to Yomi

You unlock Kensei mode once you have completed the game.

This mode is easier and more difficult at the same time. Every enemy except the boss dies with one attack. At the same time, you yourself also die after one attack. The bosses have the same life points as on easy or normal (?) difficulty.

Normal enemies are easy to defeat with this mode. Block their attack and perform a light attack.

Ranged units and spearmen are the most difficult opponents in the mode. You can dodge projectiles by rolling . This works well with archers. I find the cannoneers more difficult. They are usually covered by one or two other opponents. In such cases I use the Ozutsu. I killed the spearmen with throwing stars or arrows. There are enough of these.

The boss fights are just like the achievements "Kill ... without getting hit". The bosses are easier than in Ronin mode.

You need complete the game once on any difficulty to unlock Kensei mode.
It's not really hard and actually more fun than Ronin mode. Upgrades are your ally specially range ammo capacity. You and every other enemy will die in one hit except bosses, they take a few more hits.
Here is 100% complete walkthrough guide for every achievement and collectibles in Trek To Yomi: