Forager

The fastest way i found to gain levels is by using the blood altar in the fire biome and then choosing the boon "Madness" which gives you instantly 3 level ups at the price of 1 max HP.
If you get 2 other boons than Madness to choose from you can simply close and open the game again and so the altar will be re-rolled so just repeat until you get it.
Then you can simply build some Spirit Orbs to regain the lost HP.
Alternatively you can simply build a bunch of Spirit Orbs for instant singular level ups but i found it ineffective since it takes quite some time to build 3 orbs compared to the altar method

If you don't want to go through loosing your HP you can do the Spirit Orb method.
I was able to max level without making any orbs, at the spirit forge, with what the game generously gives you.

For:

1. Completing bundles in the museum.
2. Completing dungeons/galaxies.
3. Side quests on various islands.
4. Buying from the market

Another tip for the orbs, do not use them early. Wait until later mid to late 40s early 50s it'll be a waste to use them too early

As well, make sure you're crafting your books at the inscription table for booted EXP.

You can also make wisdom potions relatively cheaply, pair that with the oblisks you can activate for easy advancement.

Like:
Quarry > spawns stones > mining oblisk > wisdom potion > mine.

Or go to enemy heavy areas like the fire biome, the mobs there give you a decent chunk of xp paired with the relevant oblisk and wisdom potion.