The best way to level up quickly is to focus on doing quests. Just about every type of quest will give you some amount of experience, usually in larger amounts than just grinding enemies.
You will want to keep Daily Quests in particular, since these change every day. Sometimes they’re pretty basic, especially once you’ve leveled up enough, so you will get a solid chunk of EXP for little effort.
Additionally, make sure to explore the game’s areas thoroughly.
Every area in the game, be it the dungeons or the hub areas, has collectibles and hidden quests for you to do.
Challenges and Puzzles
Nearly every dungeon area in the game has Challenges and Puzzles that you can complete. These are a good source of experience and resources, so make sure you do them when they’re available.
Also, keep the Operation Briefing in mind. This is somewhat similar to Daily Quests, but they don’t have time limits.
You will find loads of small objectives here, which you’re meant to complete as you play regularly. Fulfill one of the objectives and check the briefing again to claim the rewards, which usually include EXP.