Gon uses "Bloodcrazy Butterflies"—which are attracted to the scent of blood—to track Hisoka. He spends the episode observing Hisoka from the shadows, waiting for a moment where Hisoka's guard is down, particularly when he is on the hunt for his own target.
Examinees must collect 6 points to pass the fourth phase. Their own tag and their target’s tag are worth 3 points each; any other tag is worth 1. Key Plot Points Hunter X Hunter 2011 Episode 15
Watch a breakdown of the complex strategies used during this phase of the exam: Hunter X Hunter 2011 Episode 15