Dragon Ball Z Kakarot has RPG elements such as leveling up. This page of the guide focuses on the leveling up process and presents several ways gaining experience fast.
The fastest way is, of course, to complete the main missions. They are the best source of experience points - almost every battle from the main story will give you another level. The second method is to complete substories that also offer a certain amount of valuable experience points, although certainly less than the main story.
If you want level up a bit, a good way is to get instant victories. Look for opponents who are 3 levels below yours to defeat them instantaneously. To do this, fly into the enemy at full speed and that group of enemies will be defeated immediately - you will gain EXP and rewards from this battle. This is especially effective at higher levels when you go back to previously explored locations.
You should also hunt for Villainous enemies, enhanced opponents who are worth a lot of experience points. You have to fight them if you want to unlock two hidden battles with very strong bosses.
Finally, you can eat and get bonuses from the Community Boards. This will give you additional experience, however, the methods described above are more useful. Fight a lot and get passive bonuses from the Community Boards. Fight lesser enemies, and you will quickly reach a very high level, making your character very powerful.