Much like leveling from 1-20, you will want to make sure you are completing the main story objectives and side quests. You will also notice the NPC Aimee, who offers you repeatable quests at various outposts. Not only will these provide EXP, but you will also be rewarded with Ancient Fragments.
You will want to make sure you are enhancing gear of Rare quality or higher, and evolving it after enhancement level 7. Just be careful that you are of an appropriate level to use the evolved item, as the level increases with each evolution. You will also want your main deck of Astels to have a 2-star rating or higher.
If you have been crafting and gathering during your leveling process, you will want to make sure they are leveled to at least 21, before you proceed to the level 20+ leveling areas.
It is advisable to do the highest level dungeon you can access, unless you are targeting a specific Astel Card. You can see the common Astel Card Rewards via the Dungeon menu.
- Crimson Rock Valley - Level 20
- Arau Cave - Level 25
- Hall of Equality - Level 30
- Macarzan Basin - Level 35
- Temple of Blessings - Level 40
You will still want to use Food, HP/MP potions and Astel HP Potions, but you will have access to stronger HP and MP potions, as well as Astel HP potions, from the General or Misc. Merchant in the new leveling areas. If you have been crafting your own food, once you surpass level 21 Cooking, you will be able to produce food with stronger bonuses.