The arena cut-off score has increased considerably since the latest update IS made early December 2019. Starting in version 4.0, players will get +12 points per game as long as they use a bonus hero to defeat at least 1 enemy. Prior to the update, player will get +3 points for every enemy that the bonus hero defeat. Maximum extra points per game remain the same at +12. However, since player has to defeat only 1 enemy with bonus hero, it has negative effect to the overall score.
Since it’s much easier to get the +12 points, the majority of players will get this extra points. The variations come from the base arena team score and how much the player are willing to fish for the higher score. Skill becomes less important in this mode now. The new change benefits less skillful players that invest more money (more wisely) in the game. The armor meta is now making a comeback. As of today, (Duo?) Armor heroes score the highest.