Zombiemancer Baron Raise Zombie Necromancer Summoner Ascendancy, Bandits, and Pantheon Powers
Ascendancy (Witch – Necromancer)
Mindless Aggression
While Mindless Aggression isn’t the sickest of Ascendancy nodes, we want to pick it up first as it bridges the path towards Unnatural Strength. It still gives us a little bit of Minion Speed, Minion Damage, and some Minion Attack Speed. By pathing towards Mindless Aggression we’re able to pick up Unnatural Strength after Cruel Labyrinth. Those 2 gem levels will scale your 
Raise Zombie very well!
Unnatural Strength
When it can be used this is probably the most powerful notable that the Necromancer Ascendancy offers. Obviously, we are presenting just a Zombie guide here but in general scaling Levels on Minion gems is outrageously strong. With the addition of all the extra levels to Spell Skills, Minion Skills, and so on Minions saw a huge rise in popularity as they gained tremendous amounts of power. After picking up this Ascendancy node you’ll see an immediate increase in power both for your Zombies as well as whichever other Minions you are using at that point to support them.
Commander of Darkness
Raise Zombie scales the best off of Gem levels, as mentioned, but that only scales their Physical Damage. Since we take so many sources of Gem levels, making our Zombies somehow attack faster (
Flesh Offering, 
Feeding Frenzy Support, …) is the best way of scaling them further. Commander of Darkness grants us around 15% Increased Attack Speed to our Raise Zombie as well as some extra damage and very necessary Elemental Resistances for your character so you can focus on other types of suffixes such as “% increased Strength”, “% increased Attributes”, or Flat Strength.
Mistress of Sacrifice or Bone Barrier
Bone Barrier is the bread and butter pick and was almost always used ever since the rework of the Necromancer Ascendancy. However, since its nerfs and the introduction of Glancing Blows and its interactions with Mistress of Sacrifice and 
Bone Offering one could start thinking that Bone Offering is a mistake. Bone Barrier gives you an extra Skill to use which functions as a stronger 
Steelskin. It gives you and your minions about a 2K Life buffer when its up. On top of this skill, it gives us 10% Physical Damage Reduction and 50 Energy Shield Regeneration. All in all, we do feel like Mistress of Sacrifice in its current state is stronger. Giving you and your Minions maximum Block and Spell Block chance, as well as granting your Zombies and yourself Life when you Block is just so incredibly strong.
Bandits
There really only is one option here. You require none of their benefits so we should be killing all three of them for additional skill points off of Eramir.
Pantheons
Major Gods
Soul of Lunaris is probably your best pick. However, it’s recommend that you upgrade this Pantheon by putting a 
Divine Vessel in the map device alongside a 
Palace Map. After defeating the Hallowed Husk, you’ll be getting a filled Divine Vessel in your map device which you can trade in to Sin in the Epilogue for your upgrade. This upgrade makes you immune to chain, which is very annoying to deal with if you’re playing as a Summoner.
Minor Gods
This is really just preference. As long as you don’t use Abberath’s, Ryslatha’s or Yugul’s Soul, pretty much all of them are acceptable. For pure boss kiling, Soul of Tukohama is probably a little bit more beneficial then the others.
Upgrading your Pantheons
Whilst upgrading your Pantheons never hurts you, it’s not that important for this character (besides the Upgrade for Soul of Lunaris that grants you Chain Immunity). If you have nothing else to do at some point, sure, why not, but it shouldn’t be something you focus on or spend a lot of time on.
Pantheon powers can be upgraded by using Divine Vessel in the Map Device while you are activating the particular Map that contains the boss you need. Once you’ve killed the boss and captured his soul, you need to deliver the Divine Vessel, which’ll be filled up in the map device, to Sin in Oriath.
Main Page / TL;DR