Killua is an NPC located on inside of the Japanese Fortress with Blossom trees. He'll give you the quest to obtain his spec, named after his first name "Killua".
- Killua: Assasination is a key aspect that's held at high regards within my family. Do you wish to be a successor of such tradition?
- Player Options: Yes & No
- Killua: You'll have to mentally farm yourself in order to meet the criteria, still think you've got the mental capacity?
- Player Options: Yes & Nevermind
Saying Yes will give you the quest to obtain Killua spec.
Saying No will just close the dialogue and don't give you anything.
- Endure the Way of Yakuza for 850 seconds [0/850]
- Inflict 12500 Damage [0/12500]
- Dash 125 Times [0/125]
- Take 8000 Damage [0/8000]
- Use Arrow 15 Times [0/15]
- Successfully Counter with Brawler's Instinct 100 Times [0/100]
After completing all of that you will receive an item called "Yo-Yo" Use it on Standless in order to get Killua spec.
For the Brawler's instinct part of the quest, you can do it on a your friend, or use the Akira_Dev attacking dummy to finish the quest.