After finishing the game with Constructor on Inhuman Ordeal 5 I started to wonder - Is it really possbille to do on your own without bunch of minions helping you to protect & attack? It turns out - yes, it is. I'll leave a link to that playthrough, now let's see what we can do with Umbralist.

https://steamproxy.net/sharedfiles/filedetails/?id=3499319186
The game is very punishable if you take even small wrong turn in abilities, skills or perks. That's why first thing to do is to figure out strongest sides of Umbralist such as:
1. Plenty of debilitating skills that make opponents harmless and easy escapable in emergency;
2. Umbralist can be invincible thanks to the abundance of Ambush skills;
3. There are still great scaling minions (which is important for high difficulty) even though not as much as for Constructor.
We also need to respec a lot after reaching level 100 and getting godlike items. Sometimes skills, perks or ability points we take are mandatory for survival at some point of the game but pretty useless or inferior to others in the endgame. I'll cover that in the guide.

Start on Fearless
Wipe out each and every enemy you see, buy yellow items in Romani Camp before fighting Vrikolak Leader and Scarecrow. Upgrade them. Those boss fights are the first challenging fights. At this point quantity of skills is better than quality. Let's take a closer look what I picked in order:

Splintering Strike - this is main attack that will be used most of the time in the whole playthrough. In the Endgame we change it to Wraith Strike.
Phantasmal Aide + Shadow Claws. First minion but - we need this whole skill tree because
a) Minions are scaling to the difficulty
b) Katarina will have enormous damage and Shadow Warriors take care of enemies when we can't go out of Ambush
Ink Rupture Cloaked Form + a little damage
Umbra Form + Umbral Sanctuary powerup - invulnerable for 3 seconds when you emerge from cloak.
Unnatural Terror - only for Vrikolak and The Wingless Messiah for now. Later Arcane Healing and Choro-Lock would become hotkey skills.

I remembered to take Aura only on level 9 - it wasn't needed before, but on the other hand, Mark of Slow is difficult to underestimate. I'll list some milestones I wrote down through the game:

Level 16
Second aura - Wraithlike Presence. All ability points go to Corporeal Presence so we try to take maximum advantage of it (at least until we won't be killed with one hit regardless).

Level 30
Blood Mark aura - it is not crucial but it significantly saves time before we get access to more important auras.

Level 40 - Shadow Warriors must be taken first + Shadow Brutes powerup. Next few levels you will feel relieve because of how good they are.

Level 51
Mark of Vulnerability instead of Blood Mark.

Basically from level 50 only Katarina, Werewolf and Shadows Warriors are fighting for us since it's impossible to have decent defence playing on maximum difficulty. Much later Umbralist will participate in battles again after we obtain godlike items.

At level 100 you must have developed Shadow Jolt, Wraith Strike, Chono Lock and Shadow Warriors skill trees. Respec Splintering Strike for Wraith Strike.

Final Auras are: Phantom Stance, Mark of Vulnerability, Shadow Fury.

All ability points should be reset and put from Corporeal Presence to Blade Sorcery. We maxed critical damage already and with godlike items we don't need so much health anymore.

Katarina is our tank, main attacking power and one-hit killer (Touch of Death & Ancestral Rings). But until level ~ 27 all of the points should be spent on pure physical abilities - Corpus, HP boosts,
Around level 27 respec ability points to Ethereality and skill points to chance-based skills (i.e. Touch of Death, Mirage Queen, Deathless Shroud).
Mirage Queen is better to be around 10 - helps a lot against big crowd.
Deathless Shroud from 1 to 5: this 5 seconds can be life saving.
Touch of Death is always 10.
Death Knell - 10 because +32% damage bonus goes to VH, Katarina & minions.
Spirit Arc - 10 because damage based on Ethereality.
Wraith Frenzy - 10, the faster she hits, the faster she one shots.
Power of Vengeance - very decent bonus to damage. Decide for yourself how much you want to invest in it because you want other skills too.

I changed her build a little just before final boss (as you can see in the video below). But don't let that distract you - it is not that suitable for regular enemies.

Here are the places where I died or struggled so you be prepared
Igor's poisoned explosion after death. Don't run into them.
Kobold spikes' one-shot. Be in cloak form all the time and let your minions do the work
Golem's fight is incredibly tedious and easy to die
Alpha Prototype & that locations in general. Probably the worst.
Extraction RIg - no, this is too much. This is where I dropped the difficulty for one fight because you cannot be serious expecting me to beat this with Umbralist (I did with Constructor). But okay, if the game isn't balanced and fair to us, we return the favor in next paragraph.

You should reach level 100 just before Harker & Dryad boss battle. It's time to go in Adventure Mode to earn 1000 glory points (it will allow to open gold chests to get godlike items). Here's the thing - game uses separate saves for glory points and character's inventory. Main goal now is to get all godlike, some set items and upgrade everyting +add essence. Earn 1000 glory points, backup saves and go buy a gold key to open gold chest. Take everything you got, save, close the game and move ONLY "common" folder from backup saves to Documents folder
C:/Documents/Neocore Games/Van Helsing Final Cut/Savegame/Default/common. When you reload you'll see that you have everything from gold chest in your inventory but 1000 glory points still there. Do this as much as you need to obtain items. Gold included in chests.

You can see all the items in the video below, Katarina needs to wear 3 items from Crimson Terror to get all needed bonuses but only 2 Ancestral Rings are enough: chance to kill on weapon hit surprisingly working (there is pandorahero's guide on this topic).
As a sidenote: I noticed that "chance to kill on weapon hit" works differently on certain powerful enemies: it's not like they are killable at all (some bosses are) but it will usually take more time. As if they had hidden resistances that lower probability of killing them.

Important!

When at Industrial port you should avoid looting the Domovoy corpse as it drops one of the best Umbralist daggers in the game. Items scale to player level in this game so picking up it up at max level (100) will give you the max level version of the item

-from "Fearless leveling build for Umbralist"[van-helsing.fandom.com]

Perks are mostly related to Katarina but some to increase damage of Shadow Warriors and attack speed. But the lady comes first, in order:
Wraith +10% Damage, Health Regen to Katarina
Scholary Ghost Lady Katarina recieves 6 skill points
Ghost Whisperer -33% respawn time and +33% HP regen for Katarina
Spirit Guide +20% HP for Katarina, 3 ability points
Wight 10 ability points, 2 skill points for Katarina
Black Widows +10% to poison damage

You can take other skills at your discretion for now, I took useful for certain points but obsolete later Chimera Trainer, Bloodthirsty Ghost and some more, but that's the list I have after level 100 resetting:

Manifest Fury +25% damage while at max rage
Gunslinger +5% attack speed
Gunslinger II +3 attack speed
The Flash +5% attack speed, +10% speed
Bond of Fury +20% damage to Katarina while you at max rage
Fortune's Favoured Son 8 ability poitns, 3 skill points
Master Hunter 6 ability points, 3 skill points
Elementalist +5% elemental damage, +10% elemental resistances
Resistive +10% to all resistances

You may also consider change Ghost Whisperer to Mentor (3 skill points) since Katarina is invincible most of the time.

Took less than half an hour (Constructor made it for 1 hour 18 minutes).
Have fun in the game and don't suffer on highest difficulties if you don't enjoy it!