I forgot to make a screenshot of the waypoints, so I went back.
Found a Ko rune, basically a Pul and a 4 sock flail away from a Hoto - if I find them, I might as well craft one. I could use the Ko for a Hustle armor for mercenary, but he will really need the resists and cbf from Duriel's shell for now. If it was a Lem rune, I'd make the Treachery for sure.

Making screenshots for the walkthrough almost costed me the character. I could have easily sniped the organ and save&exit, but I wanted to screenshot it on the ground.
Picked it up, got surrounded, wanted to tele away, straight into a pack of spiders with amp on. Thankfully everytime I do sketchy teleports I instantly press escape right after - this might be cheesy, but it's part of offline gameplay I guess. In this case I could have exited an online game too, but I didn't want to risk it anyway.

Might have been just unlucky or biased, but Act 3 felt like I was constantly cursed, I went back to town or to a well everytime, not risking 1-hit deaths.

Some random items, not sure if the Eldritch amulet is going to be useful, but I'll stash it anyway, got close to infinite room with all these tabs and the mats/gems not taking up space.
I'm still very careful with undead dolls. I tested the explosion of one, took off approx my third health. Minions/champs/modifiers could still make them 1-hit or 2-hit me, so I'll keep the distance.

Ruined temple was a massacre, 2 bear packs and 2 ghoul lord packs accompanied Sabrina, the room was non-stop exploding. Had to ring of fire them away almost constantly.
Sewers barely had any dolls this time, so even though it took me a while to find level 2, I was more or less safe.

This is where the cheesy part begins. I attempted Travincal, but all the council members were outside. Most of them were immune, there was no way to kill the non-immunes without the rest of the members healing them up.
I made a new game hoping that this time they were more spread out. I was lucky, lured out Icefist and his minions. His minions were immune, but I could lock one in a constant ring of fire knockback while the other died, then finished the second minion.

This time I was thinking of skipping the rest of the council members, breaking the orb and go straight for Mephisto, but I prefer to do a 100% playthrough with all quests and waypoints. So I attempted to spread out the rest and I could succeed. Jumped on the main minions with the crew and spammed ring of fire on the rest so they cannot heal him. Slowly but steadily they all went down eventually.

It's actually weird that the fire immunity of Geleb Flamefinger himself was broken, but not his minions'.

Got level 88 in the Durance, put my points to vitality straight away, then Mephisto dropped a pair of Natalya's boots for which I would have had strength if I had put my stats there instead (currently 60 base +3 from ring). Well, let's hope I find +2strength from charms to equip the new boots, otherwise I'd have to reach 89 first.

My first attempt of climbing to Mephisto was very tough, full of dolls, so I remade the game hoping that they won't spawn this time. I remember with the necromancer I constantly had to remake games to see if Urdars spawned in specific locations, figured it would be the same with dolls. I was right, cleaned almost the whole lvl 2 without dolls until I found the entrance to Mephisto.

Mephisto - like anything non-immune - was quick and smooth: keep him in the sigil and spam spells, maybe throw a potion to the mercenary now and then.

Started Act 4 and decided to skip most of its content straight away and find myself a harder, better, faster, stronger Hephasto. Ran up all the way to River wp and backtracked from there.
Fortunately the hellforge was close to the waypoint again. The only immune mobs in River were demon lords, and they didn't always spawn either.

The hellforge rewards were even worse than nightmare. I was really hoping for almost anything in the middle (Fal: Obedience; Lem: Treachery; Pul: Hoto; Um: resists).

But the real reward of the hellforge for a warlock was the friendships made along the way:

• Run 1: blessed aim --> quest completed
  
• Run 2: extra fast + fana + amp --> I really wanted to bind this one, would have been great against immunes. Unfortunately I had to save&exit as he was instantly sniping my mercenary and running towards me with the speed of light.
  
• Run 3: blessed aim --> skip
  
• Run 4: holy freeze --> skip
  
• Run 5: blessed aim --> skip
  
• Run 6: conviction --> Bound him as my new demon. Hephasto the third is lvl 88, so his conviction aura also increased to 11 (-80%/-16%)

Went back to finish off Izual. Plains was full of souls, but Apocalypse took care of them while I was evading the lightning.

Chaos sanctuary took way too long, but it was doable. It wasn't dangerous, just SLOW. A pack of champion venom lords greeted me in the door, merc+hephasto dealt with them in a couple of minutes.
Vizier's immunity was broken, that was a quick fight. However, I tried to keep it safe and clean all venom lords by small packs to not get swarmed by them, that took a while.

Then Lord de Seis spawned with fire immunity and extra fast too. The doom knights' immunities were breakable, but the extra-fast+fana pack had to be constantly crowd-controlled. The recipe was apocalypse + rof + rof + rof + apocalypse until they dropped. Seis always kept running away, straight into further fire immune mobs. Then I managed to lure him back to a corner, from then on it was job done.

I had to go back to town and refill my belts with mana pots I believe 5 or 6 times to finish Infector. The plan was to cut the venom lords down one by one and keep knocking the rest back with ring of fire for the duration.
Hephasto seems to do very low damage to them. Mercenary also does low damage, but his was actually visible to the human eye. Refill manapots after venom lords, back to chaos, select a new target, sigil, ring of fire, same choreography all over again until it was just phys+fire immune Infector left. Thankfully lethargy removed his physical immunity (albeit barely I assume), but I still think most of the damage was done by Hephasto's spectral hit + lightning enhanced traits...

No issues with Diablo, I managed to turn him away from mercenary, Hephasto was invincible. The eschuta's drop is a rarer one for the Chronicle atleast.
Time for Act 5 soon, we'll see if I can still manage the final act with the fire build.