I would argue that summoning necromancers are the easiest character type to play through hell difficulty with. My experience fighting all of hell as a summoner in single-player, bosses included, was mostly a cakewalk. This is true for several reasons:
- Summoners put up a lot of bodies with a lot of health. You can get anywhere in the range from 10 to 30 allies depending on your build. This leads to incredible damage mitigation, as you will rarely be even targeted by an enemy due to your mobs.
- Your summons scale with difficulty. A clay golem may be easily slain in normal difficulty by any boss, but in hell difficulty his hit points go into the hundreds to thousands (even at skill level 1!)
- Slowing effects. Slowing and stunning are some of the most powerful abilities available in most games. An enemy that cannot move or act is an enemy that cannot hurt you. Decrepify is my summoner's lifeblood. A 50% reduction in speed cannot be ignored. Couple this with the slowing effects of the clay golem, and you will reduce enemies, including end act bosses, to a crawl.
- You will trigger hit stun often. Having more than 10 allies means you are applying a vast number of individual attacks against an enemy at once. Even though your summons weak attacks are unlikely to stun, having 10+ chances to stun per second gives you a good stun rate. Permanent stun lock is a real thing when considering the slow effects noted above.
- One point wonders. Necromancers benefit from one point skills more than any other class. A one point skill is any skill that becomes immediately, and consistently, useful and viable with only one point in it. Examples include: Dim Vision, Decrepify, Amplify Damage, any of the Golems, Revive, Corpse Explosion, and Life Tap. If you also consider the sheer amount of +skill items you are likely to find (giving anywhere from +3 to +10! to all your skills depending on your luck), the list expands to include skills like: Bone Armor and Bone Wall.
- Auras benefit you greatly. With any of the rune words that bestow an aura or the Act 2 mercenary, you will be applying bonuses on many targets, effectively squeezing more value out of those auras.
There are also some tricks to increase your overall effectiveness as an undead master. For example, there are particulars about your summon's AI that can be exploited. Normally, your summons will collide with each other, which means that only a set number of them can gang up on an enemy at a time. You can get around this with the spell teleport. Necromancers do not get this skill, but you can find weapons with charges of it on them. Keep these items. Teleport is a powerful defensive ability, teleport when you are bringing 30 skeletons with you is a devastating weapon. When you teleport, all of your minions move to your exact destination with you.
Now here is where the trick is: if you teleport on top of an enemy, all of your minions will engage, all overlapped, with that enemy. This can be dangerous with enemies like Diablo that have powerful area of effect attacks, but if you execute it right you can permanently stun lock him with this technique. For example, I defeated hell difficulty Baal without losing a summon in less than two minutes. The technique goes as follows:
- Apply a decrepify curse on your target.
- Teleport on top of your target (your minions will follow).
- Step away from your enemy.
The slow from your clay golem and decrepify (and any cold skeleton mages you have) all stack to slow Baal down to a crawl. Your minions many, many, many attacks cancel all of Baal's attacks before they happen. He will remain stunlocked until he dies. You just need to keep decrepify up on him at all times.
I will warn though that a effectively built summoner can get boring to play, as you will often have to do little aside from cast a curse or a corpse explosion and then wait for the battle to end. However, it is a safe and powerful character to play, that is unlikely to hit the effectiveness wall in nightmare or hell that other character might encounter. Summoners are mostly about learning about your minions and corralling them properly to get them to do what you want.