Spoilers ahead (of course ^^)

Level Up in Stygian: Reign of the Old Ones is simple: just gain XP.
In total you need 5.500 Experience Points to reach Level 10 - that is not that simple.

The quests can be solved in many possible ways and give the same amount of XP.
Obvious choices or solutions bring you to Level 7 before you enter the Witch House.
But there are more under the surface - I call them "Hidden XP"

In this guide I want to show you, where some XPs are "hidden" and not so obvious. So you can leech a few XP more before you start grinding in Nithon.

Hints:

• If needed, you can switch with different artifacts to raise the required skills temporarily.
  
• maxing out a skill is cool and nice but you gain more XP (during the game) when you mix the skills at a wide range

During that guide you find locations for [LOCKPICK]ing and additional [PERK]s.
These Perks are:

• Abenaki Survival Techniques (increased health gain from rest)
  
• Arkham Explorer (increased travel speed)
  
• Victor's Scientific Practicality (+2 Scientific Research)
  
• Escape Artist I (Progression Escapes can be made more easily)

• after Julian was stabbed: use Occult when speaking to the vessel (10XP)
  
• after Julian was stabbed: use Speechcraft when talking to the mobsters (10XP)
  
• you can convince the soldier in the top left corner to rethink his life choices and gain 15XP as Soldier or use Speechcraft to gain 5XP and he tells his life story.

• wax statue: use Occult (5XP) and Investigation (5XP)
  
• use Speechcraft to get a discount (5XP)

• On the broken transporter you can use Science (5XP) and Subterfuge (5XP)
  
• There are 2 safes where you can use Subterfuge (2x 5XP) [LOCKPICK]

• Room 102: Subterfuge at a small container (5XP) [LOCKPICK]
  
• Room 201: Occult ??
  
• Room 203: Speechcraft (6XP) when talking through the door, Psychology (10XP) to calm Horatio down, Knowledge (6XP) if you let him speak and don't interrupt him
  
• Room 203: Subterfuge (6XP) on Horatio's Belongings [PERK]
  
• Room 302: Occult (10XP), Speechcraft to scare Carrion Jack (10XP)
  
• Room 304: Subterfuge (5XP) at the drawer [LOCKPICK]

• use Subterfuge at the drawer (5XP) [LOCKPICK]
  
• use Survival at the map (6XP) [Perk]
  
• use Firearms at the rustic man (10XP)
  
• use Science at the right brain container (6XP)
  
• take the canister with Stanley with you and activate it from the inventory. Now you can use
  Speechcraft to gain information about Kingsport (10XP)

• use Science on a container (6XP)

• lower the initial price for the Henchmen Eduardo with Speechcraft (5XP)
  
• During the first dialogue with Cornelius use Psyschology (5 XP) and Speechcraft (5XP)

Marino

• talk with him and use Speechcraft to avoid your imagniary cig costs (5XP)
  
• you can casually talk with him and eventually reach the dialogue option to use your Psyschology or Speechcraft skill (3XP) and after that you gain knowledge (3XP)
  
• if you ask him about a hint for the key, you can use Speechcraft to avoid cig costs(5XP)

Others

• Right after leaving the attic you can talk to Hershel and use your skill in Psyschology, Athletics or Subterfuge (5XP) to become Dart-Champion (10XP)
  
• gain knowledge (6XP) when talking to the Terrible Old Man

• use Speechcraft to receive a discount (5XP)
  
• use Medicine when you deliver the methynol (10XP)
  
• when asked to slap the boy, fake it Subterfuge (10XP)

• Krogh: Answer first occassion with Occult (6XP)
  
• Krogh: Help him with Medicine (5XP)

When the police dude appears in front of the warehouse:

• you can use Investigation or Speechcraft to find out why he left his horse alone (6XP)
  
• use Psychology or Speechcraft to don't be bothered by him (10XP)
  
• next to the dude you can use Investigation at the dead horse (6XP)

Inside:

• 3x Melee a crate (3x 5XP)
  
• Subterfuge at 2 crates (2x 5XP) [LOCKPICK]
  
• Occult the altar (10XP)

• on the broken bus you can use Science (5XP) and Subterfuge [LOCKPICK]
  
• During the talk with Donna: Psychology (10XP)

• Gaining Knowledge: 3XP
  
• Use the Occult dialogue option(3XP)
  
• Use the Speechcraft dialogue option for sell cigs instead of keys only (5XP)

• Science (6XP) at the decks [PERK]
  
• dialogue option: Science (6XP),
  
• ask for his work: Speechcraft (6XP),
  
• ask for the nature of the ghouls: Science (6XP)

• to prevent a fight you can use Medicine or Speechcraft on the dude with the white suit (10XP)
  
• in the cabin you can lockpick (Subterfuge (5XP)) the container on the table

• Medicine on the Rock with Veinroot (6XP)
  
• use the Psychology dialogue option (10XP) after you select the first dialogue options

• when Wjatal is approaching you, use Athletics or Melee to prevent a fight (10XP)
  
• the following dialogue gives you the opportunity to "inspect his reaction" with Psychology (10XP)
  
• now you can talk with Chad and lie to him Speechcraft (10XP)
  
• with Survival on the Campfire you gain another 6XP and a Perk

It's story-related but you can't come back. So use Occult on Keelan to gain 6XP and Black Ectoplasm.

Use Tongue of the Dead on the corpse and put him into a suitable coffin (10XP) then [LOCKPICK] that coffin with Subterfuge (5XP)

Use Supterfuge on Dice Game to gain 6XP and a few cigs.

You should have some specific items in the inventory and then choose the regarding options during the small travel events:

• Alkohol and cloth rag + Survival (15XP)
  
• climb down (Athletics) (5XP) + Light the lantern (use kerosine) + RUN
  
• observe the herde and its behaviour (Survival) (5XP)
  
• assemble a proper storm shelter (Survival) (5XP)
  
• use auto-suggestion (Psychology) (5XP)
  
• rely on your body (Survival) (15XP)

• Encounter with Randolph Carter: use Mind or Investigation dialogue option (5XP)
  
• Dreaming Baroness: interact with the Elegant Snow Globe (9XP)
  
• Dreaming Baroness: use the Occult dialogue option (6XP)

• use Subterfuge at the altar (9XP)
  
• use Science on the broken vehicle (9XP)
  
• you can talk to the Cultist with the glasses at the center bottom and use Psychology (9XP)
  
• equipped with a Cultist Robe you can talk Cultist right before the theatre and use
  Occult ... fhtagn![/i] (15XP)
  
• the pile of dead bodies (right bottom corner) in the theatre can be investigated
  
• after the movie you can use Psyschology at the dude who is headbanging (no XP but a Perk)
  
• after the movie you can dispell the curse (Occult), kill the zombie (Melee) or disable the Projector (Subterfuge) (15XP)

• use Athletics on a pile of bodies (9XP)
  
• use Medicine on another pile of bodies (15XP)
  
• during the first dialoge with Edemavvah you can use Psychology (15 XP)
  
• after "saving" Kabbal you can use Medicibe + scalpel on his ... "Abbel " half (10 XP)
  

• steal something with Subterfuge from Marino's Trolley (9XP)
  
  Nithon
  • calm down the Lost Man with Psychology, Firearms or Melee (15XP)
    
  • give an Occult answer, that you are not surprised by that time shift (9XP)
    
  • you can remove a Black Fang with Investigation (9XP) or a Power Leech with Athletics OR
    
  • you can use a glitch and gain endless XP by repeatedly collecting a Toruk Sample with Science
    

There are many items (books or similar) you can read after you have them in the inventory.
You have to read some of them for story / mission progress, but I try to list them all and how much XP you gain from them (without Story-XP):

• A History of Arkham(10XP)
  
• Bank Manager's Notes (3x 3XP)
  
• Page from Essex Hotel (6 XP)
  
• Wilkin's Notes (6 XP)
  
• Wilkin's Leads (6 XP)
  
• Note to Agatha (3XP)
  
• Julian's Crumbled Note (6XP)
  
• Julian's Unsent Letter (6XP)
  
• Major Assistants Journal (6XP)
  
• Shipping Manifest (6XP)
  
• Dead Man's Notes (3x 6XP)
  
• A Father's Diary (9XP)
  
• Elegant Envelope (9XP)
  
• Hines Hazards Suite Brochure (9XP)
  
• Lawyer's Suicide Note (6XP)
  
• Anomalous Rock (6XP) + Radiant Anomalous Rock (3x 9XP)

• Witch House Journal (3x 9XP)
  
• Elder Thing Fragment (3x 9XP)

As stated in the foreword, during the questing and some "Hidden XPs" you'll reach Level 7.
You can provoke some encounters with the Cultist or the Mobsters but these will be depleted pretty fast. Also some random encounters at the wilderness or open campfires will stagnate drastically.

Solution: There is a place with endless encounters: Nithon.
The problem: you wont be able to rest or refill your drugs / ammo.

So ...
if everything ran smoothly (or at least: smoothly as possible) you should already midway to level 9 before entering the Witch House. Do not enter - go back and go shopping.

During the story you gain 80XP and some items give you XP for knowledge (54XP in total ).
Each fight against the Mi-Go "scouts" you gain 30 XP, so calculate by yourself, how much fights are needed to reach Level 10.

You won't need much more items, so

• craft the ammo and drugs you need
  
• sell almost every dispensable stuff
  
• invest in more ammo and drugs

Each encounter spawns 3 Mi-Gos. The (imho) best tactic is to let your companions kill one Mi-Go and hurt another one. Your main character should focus on flight forward. If everything works fine, you can escape forward with minimal damage and ammo loss.
Every encounter results in a loss of sanity, so be prepared with appropriate drugs.

As for 1.1.6 you can glitch your level into endlessness if you had maxed out your Science-Skill.
Therefore you can indefinitely mine the Toruk at Nithon: (9XP) each time .. but hey ... ^^

When you first encounter him, talk to the Outsider and use the Occult dialogue option (5XP).
When the angry mob attacks him, you can support him to gain another 5XP.

If your relationship with the Outsider continues, you can try to use Psychology (10XP) and hypnotize him. After you calmed him down, use Speechcraft (10XP).

When you encounter him the first time, you can approach the Unnamed Soldier with Athletics, Stealth or Psychology (5XP) and investigate him with Occult, Investigate or Medicine (3XP).

After a while you can speak with him and offer him a cig. Continue this dialog till he seizes the waist of your charakter.
Now you can use Psychology or [] to flee from his grip: 10 XP

During the second encounter of Sonia in the Old Eel House you can use Investigation as very first dialogue option (3XP) to gain a hush of her photo. When talking to her choose the Science or Occult dialogue option (3XP)

If I'm missing some occassions, please contact me and I will add your contribution.
Contributions and completions are always welcome