Hunts and Follower Crystals question

Searched around the forum but did not find an answer to this question.

If you use Hunts to level up an off-hero’s XP, how do you gain follower crystals? their side quests are tied to chapter levels, right?

Why would you use Hunts if that is the case, as you would need to complete the chapters anyway to maximize crystal accumulation?

To maximize exp gain from story.

Also, to easily fill your slots with diamonds to farm diamond keys