Just FYI, Diablo in normal is notoriously hard for the summon necro. Literally, the summoner is one of the best starter characters, but normal Diablo is the BANE of its existence for sure. Everyone struggles there.
Best point, as stated above, is use decrepify and keep casting clay golem on top of him. Just try it. 
This is why:
One-Hand Damage: 10 To 22 (16 Avg)
Required Level: 36
Required Strength: 25
Durability: 15
Base Weapon Speed: [-20]
+50% Damage To Undead
10% Chance To Cast Level 2 Bone Prison When Struck
5% Chance To Cast Level 10 Bone Spirit When Struck
So let’s see ……
That’s
+5 to raise skeleton
+5 to skeleton mastery
+4 to raise skeleton mage
+2 to all other summoning skills
…. the important base skills for the most important part of your army is better than +3 … I reckon if you’d had read up a bit, you just might have noticed this. Maybe.
So, evidently, you figured, well, wrong.
I can farm ANY AND ALL areas of the game in Hell with 8 total players in the game. So where do you get your information from?
I’d like to see where blizz has told you that the faster you kill the better the drops? Fast or slow, drops are NOT determined and calculated UNTIL the death of the enemy.
So, your argument holds no water.
Show me the code in the game where it says:
“WAIT! Recalculate. He killed it faster then anyone else!”
DROPS ARE NOT CALCULATED AND DETERMINED UNTIL AFTER THE ENEMY IS DEAD. It does not have a flag or anything that says, the kill started at this time and was completed at this time!
Your best option is to use Arm to Summon up your skellies and then switch to HoTo…More over Beast. Summons retain the level they were summoned at, which is why when you get a skill shrine you then re-summon everything you can.
For leveling a summoner you dump all your pts into Raise Skelly until it is maxed. Make room for Amp, Iron Maiden, Mastery, Clay Golem and you have maxed skellies by lvl 23, 21 if you kill Izzy by then.
Normal Diablo is best killed with IM and Clay Golem.
Summoners can farm anywhere.
Mages need better damage scaling.
Thank you for the responses, everyone.
My skill tree relates the following:
Summoning
Poison and Bone
Curses
Character characteristics because my character matters (level 35)
The only socketed item I have is a double axe called Gale Song, a purple and a green gem, top most level gems.
I think me needs to respect me necro 
Alright guys. I’m going to write down what you need (gear) in order to be a godly summoner necro, or at least what I’ve had for years in original D2. I was hoping that whoever linked that video they would mention what I want to now, but they didn’t.
If anyone already wrote before me what I am about to then dismiss this.
What I’ve read through the comments, I just see people that don’t have enough knowledge or just can’t afford the best gear.
I don’t want this to be a wall of text but I’m afraid it will be so quickly without anymore delay:
Skills: The only difference in skills is that instead of skele mage I max Iron Golem for it’s better survivability.
The only curse I ever use is amplify damage.
Now gear: this is the part where most will give up for obvious reasons or will have a mediocre summoner.
You need as much +skill gear and AURA gear if you want hell to be a walk in the park
My build gear: Enigma, arm of leoric, marrowwalk, spirit, shako, trang gloves, 2x soj, mara or +3 summoner amu
Now to me that is honestly very basic, the bare minimum
Here is though, what will set me apart from everyone else:
Merc gear: hire one with might aura obviously
Helm: eth andy or eth vampire gaze
Armor: Fortitude
Weapon: Pride with lvl 20 Aura
Last but not least you make an iron golem out of the runeword: Beast
With iron gol maxed out he never dies, probably wouldn’t even need to max it out but what else to put points in to?
In original d2 it happened to me very few times that the golem disappeared. That sucks.
I guess they fixed it now in D2R.
That’s it. In my opinion pride 20(yeah 16-19 aura will do until you get one with 20) aura and beast iron golem are a must. You can switch things up.
If you want to crank things up even further, switch out a soj with wisp projector. That has a lvl 5 heart of wolverine on it
Hell diablo and baal go down in 3 seconds max, but for that enigma is key.
Instead of waiting for your army to get to them you teleport on top of them.
The great thing about enigma is that you have full control over your army.
So when I teleport on top of any mob or boss, the whole army is stacked and instantly attack whoever I teleported on. This is how you can take out any mob instantly
The 3 lesser ubers, andy, duriel and izual I can kill with this necro.
With a little tweaking for survivability maybe uber tristram could be doable.
Good luck to you all.
PS: now in D2R and with patch 2.4 I’m doing the same thing with the druid. So far so good.
No iron golem, but we have heart of wolverine!
make them use magic arrow, so they deal phys dmg and a tiny bit of magic dmg 
Agreed, and proposed a buff to Raise Skeletal Mage to significantly increase elemental damage and missile speed. Also, missiles now have chance to pierce per level.
Max 4 mages
Can have all 1 element or all 4 and everything in between
Secetable which element to summon
Can cast
Frost Nova
Meteor
Chain lightning
Poison Nova
How about 1 mage like a golem? Casting 1 of 4 elements
AoE would be very welcoming.
Bro too much maths and sense in there
The devs got a headache after the first paragraph
Just say the mages need 700% dmg buff and they’ll go straight to the same conclusion of : hahaha it’s not that bad game been fine for 20 years
And go right back to nap
The Arreat Summit
Skills Tab
Summon Mages
It explains what mages have and increases to damage per level. If the spell they cast is fireball, then it will cast according to its level and as such do that damage.
Pretty sure it is well spelled out. Just have to read it and do a little math, not that hard.
Also, Diablo Wiki spells it out just a little more for you … you do need the reading comprehension of a 6th grader though.
I’ve always secretly wanted to build a skele mage necro, but you’re right they need a buff.
An army of spellcasters is one of the coolest things possible in D2. I hope you get the buffs necessary to put them in a good space.
Person A ask for tooltip damage as most spell in every game have.
You : it’s easy you just need to go to a website search the information gather it, do a little excel sheet and alt tab between the game every time you want to know… such stupid request!!
There’s no numerical value for the tooltip ingame. It just says “+ damage.” All exact values have been calculated by external parties. And thwy are not consistent with changes being made in 2.4.
Hey dumbass, there is no preset score to put in the tool tip,the damage changes with the skels levels, and with any +skill gear you add … so yeah go figure it out. its not rocket science or are you really that damned lazy
So like every other spell in the game?
The only logical reason for them not to put the tooltip is because the dmg is truly underwhelming and that’s how they want it to be
The damage tooltip scales per level for regular skeleton warriors in this way. The issue is likely the variation in damage across each element. Visualizing them all is a UI problem. However, easily resolved with an average value across all four types
I totally agree. They even die quicker. They aren’t smart enough to stay back from the fight. It’s like they have the same AI as the skeletons but they’re mages.