One of your men needs to make a certain number of personal kills to gain Legendary Swordsman event.

I find YS is the best for this.
I select a unit that will become my rout killer unit, then i take this unit along to as many battles as i can,
i sneak this unit - and sometimes a support SA around to the back edge of the map board where i estimate the enemy will rout to.

Once this unit is in place begin your attack - try to rout the enemy one or two units at a time and have your YS-rout-killer intercept them.
Usually 2 or 3 battles will 'trigger' the legendary swordsman.

A good idea to save before each such battle - just in case your YS gets suprised and you have to statr all over agin.

My personal opinion is this event can be a pain to 'trigger'.