Astromon's maximum level is based on their grade.
Ex) 1★: up to level 10, 2★: up to level 20

Astromon's grade can be increased through Ascension, up to a maximum of ★6.

Ascension is available in the Astromon inventory.
Astromon must be at its max level, and Elemental Starstone that matches its element is required.
After Ascension, the Astromon's level is resetted to 1.

When Astromon reaches ★3/★5, the Astromon will achieve a passive skill.

The next region requires you to clear all stages in the current region to enter.

The next difficulty requires you to clear all regions on the current difficulty to enter.

The next continent requires you to clear all stages on the current continent's EXTREME difficulty to enter.

The use of Supporter Astromons does not count on that certain quest.
Make sure you're using a friend's main Astromon, not a supporter Astromon.

Only variant Astromons have leader skills, and setting a Variant Astromon as your leader activates it.
Leader skill will remain active even if the leader Astromon is defeated.