When you have unlocked the Forgotten Catacombs at level 23, you should start challenging the Wartune Forgotten Catacomb as much as possible. You can simply turn on the AFK mode and let your Hero run the Crypt starting from level 1. Also the new function Blitz, allows you to run the dungeon hands free from lvl 1 onto the highest you have gone to. It costs 1k gold per level. Recommend using crypt keys for blitz when you have gone past lvl 30 in catacombs. The Catacomb is a great source of getting Gems.