Necromancer is one of the fun and safest build in the Diablo 2, especially if you are going for summoner route. It is because your summoned minions will fight or tank the incoming damage from the monsters for you. The chances for you to get hit by the monsters are almost nil. No matter what difficulty (Normal, Nightmare, and Hell) you are playing, the summoner build will breeze through the whole game with little difficulty, although it takes time and patience to do so. Also, the summoner does not rely heavily on the uber equipment to beat the game. You can still complete the game with poor equipments, although the Uber equipments will increase your killing speed tremendously.
Summoner Skill Build:
As a necro summoner, the main skill that you need to learn and max it as soon as possible is Raise Skeleton. Here are the lists of skills that you need to learn or max as summoner.
Lv20 Raise Skeleton (Max this first)
Lv20 Skeleton Mastery (Improve your skeleton health and damage)
Lv1 Amplify Damage (Your skeleton will do double damage on the cursed monster)
Lv 1 Dim Vision (Optional but Recommended)
Lv1 Decrepify (For any tough and speedster enemies and bosses)
Lv20 Corpse Explosion (Optional but Recommended)
Lv1 Bone Armor (Optional)
Lv 1 Clay Golem (For Meat Shield purpose, also it can slow the monster/boss when it hit them) Lv1 Golem Mastery (Improve your golem)
Lv1 Summon Resist (Improve your summon resistance)
Lv? Skeleton Mage (Dump the remaining skill point if you want)
Lv1 Lower Resist (Optional)
So, why Lv20 Corpse Explosion? Well, it is because this skill is invaluable when it reaches a certain level. A high-level corpse explosion can clear the whole enemies on the screen in no time. You may argue that Lv1 Corpse Explosion is sufficient, but trust me, the result of Lv1 Corpse Explosion is pale in comparison to Lv20 Corpse Explosion. After all, it is not like you have other potential skill that worth to max once your maxed your Raise Skeleton and Skeleton Mastery.
As for the Dim Vision, this skill is a lifesaver, especially in the higher difficulty. It basically can shut down all the monsters from rushing toward you in the game. Extremely useful if you are fighting against a ranged monster or spell caster or any monster that can attack you from off-screen. You only need 1 point for it and leave the rest to the +skill item.
Revive is a good additional skill to learn. It will revive any monsters that you had slain to fight for you. The revived monsters have more HP and do more damage than your skeleton, especially if you maxed the Skeleton Mastery skill. However, this does not mean that you can rely on them 100%. It is because these revived monsters have terrible AI script and often get lost. Put one skill on the Revive skill and let your +skill item do the rest.
For the Raise Skeleton Mage skill, you may want to learn it once you completed your core skills. Skeleton mages are weak in the higher difficulty, but it can be a decent support in term of damage output for your armies. Believe it or not, Lv20 Raise Skeleton Mages (8 Skeleton Mage) can easily outdo the Lv20 Bone Spear/Spirit. So, if you plan to learn Bone Spear or Spirit for additional damage for your armies, think it again.
Str 80-100 (This so be sufficient to wear all the high-end equipment)
Dex 25 (Leave it at base unless you want the max block)
Vit 300+ (Spend all your available stats point in here. More HP, more survivability)
Energy: 25 (Leave it at base. You don”t really need it at all)
As for the character stat, never ever invest any stat point into the Energy stat. Energy stat is rather useless since all it can do is to increase your mana point. Consuming the mana potion is the best way for you to spam your skills. The mana potion is extremely easy to come by. You can buy them unlimited from the NPC shop. There is no reason for you to invest your point into the Energy stat when you can invest it to other more useful stats. If you do not like to invest too many stats point on the Str or Dex, you can rely on the charm or equipment that give additional attributes.
Also, in v1.13 version, your character now has the option to re-spec your character stats and skills. So if you somewhat screwed your character stats/skill placement or you do not like your current character build, you can reset and re-spec your character. You can re-spec your character by talking to the NPC, Akara from the Act 1. Remember, you can only do it once in every difficulty.
I may have said that Necro Summoner are safe and easy to play, but there is a certain stage or level may trouble you. Summoner”s greatest strength is to fight in the open area, because it will give your minions the room to surround the enemies and attack them. But when it comes to the narrow areas such as Maggot Lair and Arcane Sanctuary, this is a hell for Necromancer. It is mainly because your minions are unable to travel the passage effectively; thus it will lead your minions or you to fight the monster in one-on-one. So, you can expect to spend a lot of time when you are playing in these areas. You can regroup your minions through the town portal, but this does not really improve your situation a lot.
It is also common for the necro summoner to struggle against certain boss monster (Act 2 Boss, Duriel and Act 4 Boss, Diablo). In this situation, Decrepify curse is your best friend. Try to learn this before you try to engage any of them. With Decrepify curse, you can cut the bosses effectiveness by half. This will reduce their attack damage and slow their attack and movement speed considerately. Combine with the Clay Golem, the boss monster is basically moving in snail pace and unable to land any blow on you or your minions.
As for the mercenary, The Nightmare Offensive mercenary is your best mercenary that you can get. With his Might Aura, your skeletons will grow stronger and does more damage to the monsters. It is not really surprising if your skeleton in the end game can do around 1,000 damage per hit with decent equipments. If you are playing Ladder, you can access to one of the most powerful weapon for your mercenary. The weapon is known as Infinity. This weapon alone can greatly enhance your armies” battle power tremendously.
If you are playing normal difficulty, the best mercenary that you can get is the Normal Offensive mercenary. His aura will improve your minions hit rate. This will make your minions have an easier time to connect the hit on the monsters instead of missing a majority of attacks. Although some people may prefer the Normal Combat mercenary since his aura can replenish the lost health for your minions.