Hunter x Hunter Vol. 6 (manga)

Gon and Killua have split off from the rest of the group to spend the next six months training at Heavens Arena. But despite the skills they came in with, the friends are quickly outmatched by another young fighter who can access the mysterious power called "Nen." Nobody will hire a Nen-less Hunter, so Gon and Killua are determined to learn quickly. Do they have what it takes to become Nen masters?