Hunter x Hunter. Volume 20: Yoshihiro Togashi

Plucky Gon's quest to find his dad leads him into a whole world of crazy adventure.
Hunters are a special breed, dedicated to tracking down treasures, magical beasts, and even other people. But such pursuits require a license, and less than one in a hundred thousand can pass the grueling qualification exam. Those who do pass gain access to restricted areas, amazing stores of information, and the right to call themselves Hunters.
Deep in the heart of the land of NGL (Neo-Green Life), Gon and Killua have one month to train if they hope to beat their powerful foe Knuckle! But first they need to increase their baseline stamina just to get to his level. Gon discovers that his windup leaves something to be desired--it sucks up all his energy until he passes out! Meanwhile, Biscuit confronts Killua about a serious flaw in his fighting ability--one that could cost them everything!