If someone want to play user vs user for Bitcoin price prediction I recommend
https://btcbet.cc/r/iWUcbG/ which works like old btc-e betting. Unfortunately it is a service, so you have to pay 5% prize commission and you can not bet on any other thing.
The thing is OP wants to bet against a person but without an intermediary like btcbe.cc which you recommended and shamelessly flagged your referral link. This is quite different than what you recommended since it will not make use of a middleman. The problem with what OP wants is that it will be hard to grade a game by themselves, unlike just agreeing to a website.