I also play a summoner, though my style is somewhat different than Who_me. I put one point in every curse – every one is useful in some circumstances (okay, maybe not weakness). I use amplify damage most of the time, use blindness and attract regularly, (note – AI curses don’t work on uniques, bosses, champions, or oblivion knights) and only use decrepify on act bosses. I don’t use lower resistance much, but I already put 1 point in all of its prerequisites, and it’s useful when partied with elemental-damage folks.
I put only a couple of points into summon resist. I find that more than a few points in summon resist hits diminishing returns pretty quickly if you have enough +summoning skills. There’s only a 3% difference between level 11 and level 15. With a +2 all wand, a +1 all helm, +1 all shrunken head, and +3 summoning amulet, that’s +7 right there. Make the head and helm +2, and throw in a +1 all armor and an annulus, and you’re up to +10.
I maxed skeleton mage, since I had already maxed skeleton mastery and skeleton warrior. They don’t appear to do as much damage as the warriors, but they make ranged attacks and can play meatshield to spread out damage done to your minions. Plus, they aren’t hurt by iron maiden. I also put 1 point into revive, bone armor, and corpse explosion, as well as the necessary prerequisites. Putting more points into corpse explosion doesn’t increase the damage, just the radius. Revived monsters only last 3 minutes, so you don’t want to put 20 points into it, but since you need to max skeleton mastery, revives will be quite useful. And bone armor gives you a bit more survivability. Check out http://www.d2items.com/skills.php for a nice skill planner.
Killing Diablo before you reach level 30 is tough for a necromancer. You won’t have enough points in summon resist to keep your skeletons alive, and you won’t have much of a golem or an attack that does nearly enough damage. Either party with someone or keep resummoning your clay golem, decrepify Diablo, and keep your mercenary alive at all costs. If you have too much trouble keeping your merc alive, grab a bunch of mana pots and some stamina potions, too, keep your clay golem up long enough to shoot bone spears at Diablo, and evenutally you’ll take him down.
I completely agree with Who_me on mercenary strategy, but want to add that in normal, get a combat or defensive mercenary (prayer or defiance aura). As far as stats go, I like to bring energy up to 50, strength up to 84, leave dex at base 25, and put everything else into vitality. If you want to use the full Trang set, you’ll need to get your strength to 106 (see http://www.battle.net/diablo2exp/items/sets/sets7.shtml#trangouls)
As far as Marrowwalks go, this says that the charges are for bone prison, not bone wall. If you want a bone attack, bone spear does less damage than bone spirit per shot, but does more damage per mana point and can affect multiple targets if you line them up right. I almost never use a bone attack, though; only when I have very few minions and I’m up against a group of enemies where I can’t put an attract curse on them.
When I do die, it’s usually a lack of resistance due to wearing almost all mf gear.