These are the main story dungeons which you’ll have access early on. It’s recommended to tackle the easy and normal story Missions since you’ll get Crystals and other valuable rewards for first time completion. The Hard Mode version is better tackled when you already several elemental parties with dedicated SR and SSR members. Nevertheless, it’s worthwhile clearing adventure dungeons to get valuable rewards.
Aside from the standard Rank EXP, Gold, and other rewards, completing an adventure dungeon will also award you Stars. You’ll get 3 stars for completing a dungeon without anyone from your team dying, and lower stars depending on your team members knocked out from battle. These stars count towards the Star Collection List, which will allow you to collect additional rewards by reaching the target star thresholds. If you happen to get a lower star rating on a dungeon, you can always come back and clear it when you have a better and stronger team.