Hidden Quest is Easy!

It's about reaching a level, but just doing the daily quests << makes it go up so fast. You can also level up by repeatedly clearing stages (consuming energy), but just in case... For the level right before achieving the hidden quest, push it with daily quests. I once leveled up by repeatedly clearing hidden quests in another game and couldn't achieve it, so I contacted support... Anyway, you'll reach level 15 quickly, it's easy, so I recommend it.