You can play arcade mode to get a lot of LDs. Check https://www.urban-rivals.com/game/gamemodes/arcade.php for further information.
But if some or all remaining seasons are not alraedy unlocked just wait some (and don't spend credits to unlock).
It was said that Arcade mode is going to be free.
Another issue with that system was they inadvertently made a pseudo Cr market since the old retired artworks were only obtainable by market/trade and not by event or pulling from the cr distribution bonus system at the time making the retired artworks even rarer than Cr cards since the supply can only get smaller. so a lot of people stockpiled the hell out of them to only be punished for their greed when UR decided to immediately convert all old cards into new art unless they invested in a card the eventually went cr or mt like GraksmxxT Mt or Ratanah Mt a lot of people lost a small fortune.