This is the best build for this weapon that I could make myself.

Disciplin's Merit helps to trigger melee affictions. Any status damage inflicted on your enemy after using Tennokai gains 6 stacks of all statuses inflicted on them on top of that. Heat, Electrycity or Slash dont have any limits for stacks and work perfectrly with this build. If you manage stack some stasuses on your enemy, after tennokay he will explode with huge dmg from 1.5 mil to 5 mil just from 4 attacks of the weapon.

Umbra mods could be changed for galvanized one and prime [Pressure Point]. It was used to make bonuses to my other umbra mods in [Excalibur].