How to do a quick trade

The hidden quest is level 60 and costs about 15000 gems. The important thing is the main quest, so get money by auto-battling and focus on upgrading first. Run invasions, collect coins, and buy that cat character from the invasion shop. It makes the game easier. Then, you can complete it while doing the level 60 main quest. Once you're done, you've succeeded in a quick trade.