Taken from this thread in the HGL subscriber section:

http://forums.hellgatelondon.com/showthread.php?t=70538

***SPOILER WARNING***

What follows is a guide to the big fights you'll be facing on the path to Moloch, along with a minor preface on getting into those fights. This post will be updated with additional information as it becomes relevant. This information is provided mainly with the intent of aiding the hardcore players out there to prevent unfortunate incidents of characters being one-shot unexpectedly (of which there are quite a few places where such is possible). Knowing is half the battle. If you wish not to know the secrets of these demons, navigate away from this page with great haste and look upon it no more.

The Essences
So to get access to the caste caves you need 5 of their respective essences. Below is a hardly complete list of places you can go to acquire these essences off of champion (rare/legendary/named) mobs. Each such mob you encounter has a 15% chance of dropping an essence (per Scapes). This one section is the one I see getting updated the most, so let me know where you reliably find your essences:

I've listed the named champions that are farmable at endgame (i.e. do not require a quest to find) in yellow. For a more exhaustive listing of all unique mobs that can drop essences, refer to MadNerd's List of Unique Monsters.

Beast
St. Paul's Hellgate
Hell: Rash
Ploughyard
Upper/Lower Kingsway Sewers: Sleestak, Big Ben
Charing Cross Approach: Ember Freak

Demon
St. Paul's Hellgate
Hell: Sydonai, Discord
Ploughyard
Covent Garden Market: Shulgoth

Necro
Necropolis
Tunnels/Access Ducts

Spectral
Hell: Flaw, Bedlam
The Downed Exospecter
Ploughyard

The Caste Caves
Once you have your 5 essences of the desired caste (or 5 of all four), you can proceed into Wake Hallow and start unlocking the caste caves. The one person holding all 5 essences is the one that does the unlocking by clicking on the torch of the correct color (refer to the essence in your inventory) next to a locked portal. This portal will remain open for the person that used their essence until you pick up the boss head (per OldBrownShoe). Using the torch will mark you as the one to receive the head of that cave's boss when it is killed, meaning that you will be the only one to see the head of the boss. Speaking of cave bosses, here's a brief rundown of what to expect for each caste:

Beast - Ash
Located in: The Maw (Blood Walk)
Archetype: Leviathan Fiend
Attacks: Napalm Run, Fireball
Strategy: Strafe out of the way when Ash flies at you to avoid getting hit by the napalm stream. Melee classes will have more difficulty with this boss as it does not engage in melee attacks

Demon - Errgoth
Located in: Hell's Heart (Hallow Deep)
Archetype: Shulgoth
Attacks: Mack Truck strike, Orbs of Death
Strategy: Fights like Shulgoth, hits harder, and when he performs his breath attack he releases purple orbs of pain (take a volley from Syd and condense it into a few orbs).

Necro - Oriax
Located in: Lair of the Damned (Skull Step)
Archetype: Zombie Summoner
Attacks: Glimmering strike, Summon Bruiser
Strategy: Handle it like you would any other summoner--it can't summon when it's dead. The issue is dealing with 8 Bruisers either smashing your face or launching spiked explosives at you. Handle with care and keep moving. As a special danger, this summoner also possesses the attribute of a Fellbore Striker, except with a Glimmering strike as opposed to a napalm strike. This attack, unlike the striker's attack, has no symbol on the ground and does an excessively large amount of damage. Like the striker's napalm attack, you can hear this one coming if you listen.

Spectral - The Nicor
Located in: Torment (Shadow Pass)
Archetype: Tortured Soul
Attacks: Warping
Strategy: This particular boss is also surrounded by similar tortured souls, so if you stroll around a corner (if you could call anything on that tileset a 'corner') too fast, you'll end up with half a dozen souls warping through you. The boss in particular does far greater damage than its entourage, so tread carefully and pick off the group slowly if possible.

Moloch's Step and The Necro Fall
This is where things change up. Like the beast tileset, Moloch's Step is outdoors, and you are faced with every caste along with winged demons flying in for added enjoyment. Getting through this area is straightforward enough if you've been clearing Wake Hallow. Once you get to The Necro Fall, take a moment to take in the massiveness that is Moloch's Lair (take necropolis and make it one multi-level area). This single area is the one point at which I have to take pause and wonder whether or not I should ruin any surprises for those who have not yet seen it for themselves. Regardless, I shall continue. After clearing through The Necro Fall you are finally faced with Moloch himself, which I won't go into too much detail about.

Demon Overlord - Moloch
Archetype: Pit Baron
Attacks: Tail Swipe (a la cinematic), Fireball
Strategy: Protip: Avoid the demon's attacks to avoid dying.

...and some screenshots:

cool guide

my tip for summoners VS Ash
: keep spamming shooter elementals to prevent Ash's shield recharging (as he is really fast and might be hard to hit with your own weapon continously)

thats if you dont mind me giving tips

thats one tight looking gun

Not at all; i havent done Stonehenge yet so have at it


That's just a sniper rifle, isn't it?

I lol'd

Screenshots are sweet, I might have to become a founder/sub.

Up so this doesn't get lost

Huge changes have been made to Moloch, so you might want to add notes about that. He's much harder to kill now and has new skills.

I think this is the coolest screenshot of me just chilling in midair while on the way to moloch

nice tombstone in that shot. wonder who that was