Imaculata
Hero
In this case though, it is definitely also a case of a DM not being properly prepared for the strength of his party. The DM should have been aware that Force Cage was a spell his players could cast, and even if he didn't know, he should have found a way to allow the dragon to escape the spell (even if it costs a legendary action to do so). Also, and this is very important:
Always have minions support your big bad boss, and have more than one objective for your players during combat!
Always have minions support your big bad boss, and have more than one objective for your players during combat!