Physically Immune manosters will not be an issue if he has Amplify Damage.
My suggestion would be to place at least one point in all the curses. The versitality of a Necromancer's curses are something that can easily change the tide of battle within a second. If nothing else, be sure to get Amplify Damage, Dim Vision, and Decrepify.
This ought to leave you with around 50 or so skill points. Here, you basically have four choices, which are as follows:
Golems
Make use of a golem, such as Clay or Iron. Clay Golem will be the ultimate tank, as it absorbs physical damage, has the most life of all the golems, and slows the enemy by a substantial amount. An Iron Golem can either be used as an additional aura or as a tank that is immune to all types of elemental damage and/or a minion with a ton of Crushing Blow (Hone Sundan). A Fire Golem is nigh useless, and a Blood Golem will only kill you.
Choosing the golem route requires you to spend points in Golem Mastery. Although the Blood Golem offers a synergy of life to other golems, it's quite literally useless, as it only adds around 200 life.
Poison Damage
Another route to take is travel along the Poison path. Your destination here would be Poison Nova, which makes a great supporting spell to speed up the killing of your minions. Poison damage also halts monster heal, so this in itself is useful. Simply be wary that many enemies in Hell difficulty either have high Poison resistance or are immune. Lower Resist helps, but then you must ask yourself what matters more. Your poison damage, or your minons's damge output? If the former, use Lower Resist, if the latter, use Amplify Damage and Decrepify. The nice thing about this route is that you can almost maximize your Poison Nova and have a substantial army. Additionally, casting Poison Nova and whatnot is much more interesting than simply casting Amplify Damage and nothing else.
Bone Spear and Spirit
Another route to take would be in the vein of a Bonemancer. However, I strongly suggest not taking this route, as your Bone Spear and Bone Spirit damage will be utterly pitiful - which in turn spells trouble or stalemate in hell difficulty. This is because it takes a gargantuan amount of skill points spent into synergies and whatnot for either of these two spells to be useful.
Skeletal Magi
The last route to take would be in maxing your Skeletal Magi. There is a persistent debate as to whether or not this is of any use. This is because they don't put out as much damage as Skelton Warriors, have less Life and Defense, and when you have enough +skill equipment, only one point will often net around 10 Skeletal Magi. However, a couple of things to note:
- The poison damage from the poison mages will halt monster heal.
- Lower Resist will increase their damage. Used with conjunction with your Corpse Explosion and/or Poison Nova, this can do considerable damage.
- The larger your army, the better. A summoner's strength comes in the size of his army.
From personal experience, I will say that I hardly ever place more than one point in this skill; however, having a large number of Magi does make Boss fights like Diablo Clone and the Ubers
much, much easier.
Hope that helped.