[District]

I'm looking for an old-anime, I remember that in one episode a little boy and a slightly older boy were having a challenge in an academy to get a rubber ball off of a master. At one point the slightly older boy used some advance illusion technique to get it. Basically it made it look like a bunch of clones/shadows behind him wherever he moved. He was kinda stronger I think. I'm quite sure this is a much older anime.

It had something to do with assasins.

Please help me :(

Edit: It was HunterxHunter. Thanks for the PMs everyone.