I was playing Arena as a bronze league player and getting matched against opponents that were in the elite league. The limitation setting (found on the game's page) for opponent player level (ELO?) does not apply to arena matchmaking. Instead you have to navigate to a hidden settings page to limit your arena opponent player level range. This limitation setting for Arena is in Play Now -> Arena tab -> Click gear icon next to the game of choice -> Show advanced options -> Player's level. Please make this easier to find or allow the usual limitations to apply in arena mode. 
If you would like to see this change implemented, please click this link and upvote my suggestion: https://boardgamearena.com/bug?id=122779 Thank you! 
If you would like to see this change implemented, please click this link and upvote my suggestion: https://boardgamearena.com/bug?id=122779 Thank you!