A big part of progressing in The First Descendant is leveling character, and gear. Doing so levels perks, abilities, allows for more modules, and more to help overcome the challenges that await. However, doing this can be a mixed bag. Later content is easier to level, though finishing it with these characters can be rather difficult. Likewise, low-level content is easy to do, though the actual value is rather low. Thankfully, there is a simple way to gain a lot of levels rather quickly. – IS

Source: N4G PC How to Quickly Level in The First Descendant