Leveling up your chosen Descendant in The First Descendant is the best way to upgrade your character's stats. The leveling system in this game is quite simple, at least for leveling up your Descendant and not your Mastery Rank or Weapon Level. If you want to level up your Descendant fast, I have a few tips for you!
The First Descendant FAST XP Method Fast Leveling Guide
Since you can level your Descendant up to level 40, it's best to find a happy medium between leveling up quickly and leveling up in a way that's still fun. These five basic leveling strategies will help you enjoy your game while maximizing your Descendant's level:
Bosses have yellow titles when you complete battlefield missions, as opposed to the red titles of regular Vulgus enemies. When you complete dungeon-style missions (for more information on these missions, scroll down to number two on this list), they have red titles but have much more health and can heal themselves two or three times. They are much harder to kill, and it’s best to team up with other players during missions to take them down faster.
Since they are bigger, badder enemies, you get more EXP for killing them than Vulgus. However, it's not the best technique to use, as the time it takes to kill some of them is ridiculous.