Instructions
Invest all your skill points in the "Poison and Bone Skills" skill tree with the exception of one point, which you should invest in "Amplify Damage" in the "Curses" skill tree. Put one point into "Teeth," then max out "Bone Spear", "Bone Spirit," "Bone Wall" and "Bone Prison." "Bone Spirit" will be your primary skill, and it receives damage bonuses from all of the other spells listed. This is the highest damage spell a Necro can have.
Make your own or trade for an Enigma, which is a rune-word item made by placing "Jah", "Ith" and "Ber" runes into a piece of three-socketed armor. Enigma adds two to all your skill levels and grants you the ability to teleport. The economy in "Diablo II" is constantly changing, so you are going to have to barter for the components to make one or trade two to five high runes for an Enigma that already has been made.
Make your own or trade for a Heart of the Oak, or Hoto. You make Hoto by placing the runes "Ko", "Vex", "Pul" and "Thul into a four-socketed flail. This is one of the least expensive high-level rune words that you can make. Hoto grants 3 to all skill levels, a faster cast rate, and 30 to 40 to all resistances.
Equip your character with any other gear you can find that adds to all resistances or skill levels. A Hellfire Torch and an Annhilus charm both add to skill levels, and Annihlus also adds to experience gained and all resistances.