Duel Mode is open.
In the game, switch from Arena to Duel. Line up three characters and press Fight. You are matched with a player near your rating, and you only see who it is when the fight starts. Your first fights theirs. The winner stays in with the health they have left, and the side that runs out of characters loses. The same team defends for you while you are away.
Every fight you start pays stones and gives those three characters XP. You get 100 fights a day. Beating other players raises your rating, and the top 10 sit on the leaderboard.
Copies come from packs, and no two of the same character are quite the same. Merge two of the same rank to rank up, and use Synthesis when you want to push a copy further. Unlock a character in Soul-Arena first, then here with stones. Starters can fight right away.
Speed beats Power, Power beats Guard, and Guard beats Speed. The help screen inside Duel covers the rest.