Heavens arena is where victorious fighters ascend to higher floors as part of its reward system and just like you many others are past the 200th floor in heavens arena. You had a fight with a boy your age, named Killua

In the fight you used your nen ability but the boy didn't use his, so you thought he didn't know about his nen but reality he just found out his. Using your nen you beat him with ease

after the fight walking out to the floor's lobby to find him, you see the boy with another boy, Gon, as well