For this achievement your overall party level needs to be at 20. You can see your current progress after every battle - the top bar above the characters is the one that is important. In my case the overall level got to 20 just as my last character got to level 20.
Like mentioned by "Vhufz" I selected the Abyss mode which seems to be endless but the floors are random so you might get something difficult if you are unlucky. I was on the 10th floor when this achievement unlocked for me, but it might vary based on how much XP you get. I don't have a recommendation for your party setup but personally I was quite lucky having the assassin in my party.
Some things you can consider to improve your chances:
1. There are some random tiles which offer you XP or coins - XP obviously helps you with this achievements.
2. Try to fight all enemies on a floor before fighting the boss to earn as much XP as possible.
3. Use the rest function
often to have your party rest and heal them
.
4. Sometimes the game can crash (happened to me once during my run). While you might be lucky to continue where you left off, it's sometimes safer to "save & quit" after a boss fight and continue from the main menu again to make sure that the the game saved your progress.