What you need:

Basic Fire Spell – Fireball is enough. You don't have to be a mage—even if you can't learn it, just buy a scroll. Best if the scroll was brought in the Black Market.

Mana camouflage ability—if you're casting your own spells, you can get it at the Black Market in the earliest levels. Talk to a hidden NPC near the entrance named Shadow Cauldron. He was invisible but when you hover your sight around, his body would be highlighted. He'll teach you the skill for free as a commendation.

Disguise. You can get the skill by registering at the Thieves' guild, also located in the Black Market. They're accessible from most sewers and contrary to the popular belief—it's a perfectly safe place as long you didn't do anything like attacking NPCs or stealing their items.

Ability to remove your own tracks, talk to the NPC at the Thieves' guild that's holding a whip and they'll teach you for free after you've become a member.

A flammable agent like oil for insurance. They can be bought at item shops.

Some luck (dry weather is best, avoid rainy days. Look for ordinary forest without tree guardians or stupid elves.)

Willing to travel quickly from place to place.

Good to have:

Hatred / apathetic towards nature and their races like Elves and Fairies.

Means of fast movement—best if it didn't leave any tracks or mana leakage. In early levels, you can unlock the Nightmare if you're starting at Nifelheim. Visit Graveyard of Siddhis and talk to Ol' Hargraven about the rumors.

The Quest doesn't have any level limitation and there's no combat. You just have to follow the storyline before you'll get a free pet.

You can also unearth a Floating Dwarven Construct if you start in Dunmar.

With both the Nightmare and Dwarven Construct, you can assault all the ordinary forests without getting caught.

...but as long as you have mana camouflage, disguise and ability to remove tracks, it should be easy enough to escape the detection.

...as long the one you assault is just an ordinary forest.

...

How to:

Wait until the weather is suitable and go to an ordinary forest.

Pour oil at the first trees in close proximity.

Cast fireball / use the fireball scroll at the oiled tree.

Quickly run with your means of transport and go look for another ordinary forest.

You should gain lots of experience and charcoal drops straight to your inventory. Plus, if any beasts were slain during the fire, you'll also get the experience and drops.

Repeat the earlier steps until you gained enough.

Won't be long 'til you gained the title [Forest Menace] which grants you extra damage to its inhabitants.

You'll also gain [Arsonist] title, which further increases the damage of your fire attacks, especially against buildings and constructs.

Recommended for anyone who doesn't give a fuck and loves fire.

...

Q/A

Q: If felling trees are going to give me tons of exp, isn't it better to be a lumberjack?

A: But it's a lot of work to cut the trees.

Q: The preparation seems like a lot of work as well.

A: The payout is worth it, unlike a lumberjack.

Q: You shouldn't wreck nature.

A: The forest would just return to its original state after a week. I'm just farming EXP.

Q: Have you ever get caught doing this? How about your relationship with Elves and Fays.

A: Just give them money. Everyone loves money. Fays especially love shiny garbages. Their affinity should be reset to zero with just a few golds. Bribing is broken.

You can even kill someone's entire family and just compensate with coins, they would let you off as if nothing happened.

How to get lots of gold?

You know a store with a second floor?

First, you made a mark on the board, which was visible from the first floor, just in front of the store. Then go downstairs and walk a few steps in the store's direction.

While at it, try looking up. There should be a [Hidden Chest] highlighted by your interface. Put every item inside to your inventory.

For the maximum amount of lulz, you can even sell it back to the same shopkeeper for easy cash. Notice, their inventory would be empty by then.

Or you can go sell it to fences. If you had no idea, just talk to a random shopkeeper with animal heads.

Q: Didn't you say you want to stir trouble with Elves and Fays?

A: I neither hate or love them. What I care about is leveling fast. Getting stronger the fastest I could while having the most fun.

Q: How about leveling the normal way?

A: It's boring. If I have to level up, it's better to cause as much pain and suffering to others for my own benefit as well.

Q: You're a terrible person.

A: Thanks for the compliment. Good day.