Nowaru

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He needs to run to the enemies for 2-3 rounds if they don't move forward themselves (and mages and archers won't).

The attack range of an average rifleman or mage is equal to 10 cells, which is exactly the character's speed.

It turns out that you need to take skills in movement and invest a lot in agility, sacrificing attack and defense.

You don't need a lot, 20 is more than enough. Besides, agility is very useful as a defensive stat.

And to perform a decapitation, you need to invest a lot in willpower, otherwise you won't have enough mana.

25 willpower is more than enough. In theory, you can even have less, but I have a fondness for numbers that are multiples of 5.

Since accuracy is our priority, strength will be allocated based on what's left (health is at a minimum anyway).

Strength is not needed at all. Why would it be?

As a result, they will cast spells before you can reach them.

You can parry spells. And you can also dodge them.

Then, without mana for decapitation, it's completely useless, and they will be killed in both melee and ranged combat.

Hint: "Reaper" and "Tranquility" are not taken just to sit idle.

Added 3 minutes later

but the unwillingness to be a samurai is a matter of principle.

Well, then Shameimaru is not an option at all, in my opinion.