This is what I got from support
Hi there,
Thank you for contacting Disney Heroes’ support regarding Arena.
Being a fellow player, I certainly understand your concern. Sometimes players get more points with fewer Battles.
To avoid this happening we do have split and merge mechanisms in Arena or Coliseum which match players based on an algorithm. This is a system-generated process and we do not have any control over it. I request you wait a bit more and see if a merge happens.
To avoid a cluster of Players stagnant in the Higher Divisions, it’s important that the lower divisions be arranged accordingly. The algorithm for these promotions has been set accordingly so that our players do not feel too much time stagnant in the same division. Our development team is working on improving this feature.
I appreciate your patience and understanding in this regard. If you have any additional queries, please feel free to get in touch with us and we are more than happy to help you.
Thanks for playing Disney Heroes! Have a nice day