So I toyed around a bit with the baal-running necromancer.
First I tried the plain summoner build that Tupsi has been doing well with and then I decided to give a variant of usamahanas' build a shot:
Essentially a poison necro with 1 pt in all curses, 1 in iron golem+summon mastery+skeleton mastery and maxed raise skeleton. The remaining points can be invested in either CE, Skeleton Mastery or Lower Resistances, CE being the clear favorite in my book as you gotta make sure the few corpses you get in baalruns will hit as many targets as possible.
I aimed for 50 FCR and then maximizing poison damage while maintaining the essentials on other areas but with fairly standard equipment. The real difference between this build and what usamahanas used to work with; a much higher performance merc because of 'Infinity', 'Fortitude' and a 'Beast' Iron Golem. In theory you then have a merc similar to what a pure summoner has while the skeletons are weaker (but still reasonably decent) while you have a 9-10k PNova and in this case -126 EPR. I didn't try out the original 3-parts Trang-Oul's that usamahanas recommended because it puts much strain on your already limited resistances while making it harder to reach 50 FCR which I consider the minimum.
If you have a +2 or +3/20 FCR/2 socket circlet you may be able to pull off the triple Trang-Oul for an additional -25 EPR while maintaining 50 FCR and reasonable resistances.
I found it very hard to go below 3:30 with this build. Much to my surprise as I think he can clean pre-waves a bit faster, kill baal a bit faster and should have less issues with Listers gang.
As long as Lister isn't aura-enchanted the skeletons survived just fine. The pack still took quite long to kill even with a 10k PNova, I would have to go back and try out the Trang-Oul combo to see if those last -25% EPR really make enough of a difference.
Wave 4 is not really as much of an issue as you might think, you play like the way you would with a pure summoner; stack your group with a teleport and hope to kill a minion or two as fast as possible to get the CE rolling.
While the build may be more reliable against Lister it definately has some cons:
a) 'Beast' Iron Golem is a bit risky with no real skills left over for Golem Mastery but I think its fairly mandatory to have a merc that can handle W4 properly.
b) Not enough skills which means 1 pt golem mastery, mediocre skeletons and possibly a lack of CE radius with no or a very weak Bone Armor.
c) Less flexiblility both in terms of skills and equipment; less room for MF in itemslots & sockets.
d) A bit more difficult/confusing to play as you have two main curses and two different attacks
Conclusion: Overall I would say it seems inferior to a properly equipped and played raw summon necro, I would have to test further with a circlet/Trang-Oul combo to be totally convinced however.
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Now with that out of the way I want to dive deeper into the optimization of a pure summoner.
Skills?
The core of 20 points into Raise Skeleton, Skeleton Mastery and Corpse Explosion along with 1 point in the OPW's leaves you with ~30 skills to spare at the higher levels. In my opinion there are two options only: Bone Wall for the Bone Armor synergy and Golem Mastery if you're making an Iron Golem out of an expensive runeword. If that's the case you should make sure you wear as much +summoning skills/golem mastery items as possible when you first summon it; ideally a +5 or 6 magic wand, skill shrine, 10 skill charms etc.
Items?
There are two aspects here. One part is what you put on your necromancer and the other is what you use on your minions - the Town Guard and your Iron Golem.
The mercenary obviously has to wear Infinity, ideally in a Great Poleaxe, while the helm and armor slots aren't completely set in stone. If you want the fastest possible baalruns you want a 'Cham' Guillaume's Face and either Fortitude or Treachery for the armor slot.
I haven't done enough sample runs to completely determine which of the armors is better as the amount of crushing blows performed on baal is fairly random. One minor issue though is that your mercenary won't have maxed resistances with 'Cham' Guillaume's and Treachery if Fade hasn't procced. If your Infinity is in a different polearm then it becomes a bit more clear with Treachery favored for the slower polearms and Fortitude favored for the faster ones.
One thing is for certain though: Your mercenary should have Cannot Be Frozen from his helmet. That is Kira's Guardian if you don't have a Cham to socket in GFace which I would prioritize over making the Pride runeword for an Iron Golem.
As for the character itself you can go with a fairly standard setup with 50 FCR that allows a good chunk of MF:
'Beast' non-ethereal
'Enigma'
Harlequin Crest 'Perfect Topaz'
Homunculus 'Perfect Topaz' or 'Um'
Mara's
Trang-Oul's gloves or Magefist
Arachnid Mesh
War Traveler
SoJ/BKWB
Rare/Crafted FCR ring
If you have the right circlet you may want to ditch the MF from Harlequin Crest and War Traveler in favor of squeezing out slightly faster runs:
Ideally it should have +2 necro, 20 FCR and the Visionary prefix. Couple this with a Ravenfrost instead of a +skill ring, a crafted pair of Gloves and either Gore Rider or Goblin Toe and you should be able to apply a decent amount of Crushing Blow to Baal yourself.
As for the golem there are a few choices with the ideal probably being an Iron Golem made out of a Pride polearm.
If you cannot afford such luxury there are some other options:
a) Use Fire/Clay Golem instead; The aura of the Fire Golem may allow you to activate the monsters in the throne room slightly faster and the Clay Golem allows you to use Amplify damage instead of Decrepify versus Baal.
b) Iron Golem made from Immortal King's Stone Crusher or similar for the Crushing Blow
c) Insight Iron Golem for the lazy as you should have enough time to pick up mana potions during Baalruns.
Gameplay strategy?
Before wave #1:
Teleport to the center of the room and spawn a Bone Wall to get the attention of any monsters, avoid getting decrepify on merc. Use Bone Prison or Attract against dolls if necessary.
Before all waves you should try to stack your minions with a teleport right at the last second and then quickly walk right to tank the Decrepify.
Wave #1:
Amplify -> target Colenzo -> Confuse -> summon skeletons from the remaining minions.
Wave #2:
Target skeletons with stack and blast away with CE.
Wave #3:
A properly timed Dim Vision will prevent all Bartucs' minions from casting Hydra but you still need to finish off Bartuc before he summons one, a scenario that is nearly impossible.
Otherwise you simply use the extra time provided by the Hydras to clear the whole wave without using CE in order to use the corpses to summon the Skeletons you may be missing.
Wave #4:
Teleport to the middle of the room. Amplify -> Terror -> Confuse when you have isolated Ventar. Possibly use Bone Prison to keep the minions away. Rejoice as you Wave 5 incoming with lots of Balrog corpses.
Wave #5:
Stack your minions and target one of Lister's Minions while avoiding Decrepify. If Wave #4 was done properly this one is a piece of cake.
Baal:
Teleport -> Decrepify -> Amplify if you have a Clay Golem and Baal was slowed by it otherwise maintain Decrepify and use normal attack yourself for the chance of applying Crushing Blow from 'Beast' if nowhere else.
Now lets see some more sub-3 minute runs!
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