Hogwarts Legacy

To reach lv 40 you need to reach 100% overall progress in the challenges section.
In Hogwarts Legacy you gain exp while completing challenges. For example, one of the combat ones is to defeat Dark Wizards. Once you finish it, defeating Dark Wizards will NOT give you any more exp. You need to finish other challenges if you wanna increase your level.

The challenges are divided into 5 categories:
Combat , Quests, Exploration, Field Guide Pages and Room of Requirement

Combat:
Defeat Dark Wizards
Defeat Dugbogs (if you need more just visit some of their lairs around the map)
Defeat Goblins
Defeat Inferi (visit lairs if you need more)
Defeat Infamous Foes (you can spot them pretty easily on the map while casting revelio on a broom. Some of them are TIED to side quests, they won't appear unless you do their quests, but since you have to do all the side quests to hit lv 40, you'll get them all)
Defeat Spiders
Defeat Trolls (if you have troubles with them, I'd recommend this video made by me: )
Defeat Mongrels
Complete Dueling Feats (Some are tricky, some are super easy. Remember that you can change spells midfight, in case they ask you to do something specific and you don't have the right spell equipped)

Quests:
Complete Assignments
Complete Main Quests (yes you need to finish the game to hit lv 40, you can keep playing as much as you want after finishing the main quest line)
Complete Side/Relationship quests (You need to finish all of them, those are also important because some of them have Infamous Foes that you need to defeat)

Exploration:
Collect Ancient Magic Traces (they appear on the map with an Icon, in case use Revelio on a broom)
Pop Balloons (They don't appear as an Icon on the map, but they are pretty big and hard to miss, especially since you have to pop 5 of them each time. Here is a guide with all the location, all credit goes to PowerPyx and the team behind: https://www.powerpyx.com/hogwarts-legacy-all-balloons-locati...)
Landing Platforms: They don't appear as an icon, but they do appear with Revelio as a huge blue disk. If you don't want to go around spamming revelio, here's a guide (all credit goes to the creators) https://www.pushsquare.com/guides/hogwarts-legacy-all-landin...
Complete Merlin Trials: You don't have to do ALL of them (that's another achievement) to finish this challenge, I think it's around 65% of the total
Find Astronomy Tables: They appear on the map with an Icon, but you need to unlock them through the main quest first
Solve Hogwarts Mystery: There are 3 Hogwarts Mystery that you need to find: The bridge one, the headmaster key and the Pendulum clock ( Credit to LunarGaming for the video if you have troubles solving them by yourself)

Field Guide Pages:
Hogsmade: You need to collect all the 55 pages in Hogsmade (I used this video for the 4 I was missing , all credit to the author of the video)
Hogwarts: You don't have to collect all of them, but a big chunk of it ( if you need more)
Highlands: There are 15 of them and they have an icon in the map, so pretty easy. Use revelio on a broom to find them if you have issues

Room of Requirement:
Breed Unique Beasts: It takes 30 mins to breed an animal, you may want to do this when you have all the vivariums available (4) to speed up the process. The easiest one are: Puffskein, Mooncalf, NIffler, Jobberknoll, Kneazle, Giant Toad, Fwooper, Diricawl, Thestral and Unicorns (since their spawn is pretty close to hogwarts and a fast travel point).
Rescue Beasts: Just use Puffskein dens, it's the fastest way since each den has 7 to 10 beasts and they are very easy to capture
Upgrade your gear: to be material efficient just do the lv 1 for green items, but you'll do it naturally.

Once you complete all this challenges, you will hit lv 40! Remember, when a challenge is over, you WON'T get any more exp for it!

Happy Hunting!