Currently level 18.
Many people will advice you to use Masters Touch spell with combination of the strongest two handed weapon you can find. In addition the undead summons will aid you well.
Personaly I didn't go that way, mostly because I think summons blow (I'm wrong, they help at low levels.) and I wasn't aware that with Master's Touch you can do some good damage as melee (At level 1-4 at least).
From level 1-6 I just used offensives spells, when I solo I used grease often.
Once I reached level 6, I took the pale master prestiege and started self healing with lesser death aura. It helped a lot, I kept going with offensive spells.
At level 7 came the turning point, firewall and death aura; run across the dungeon, grab a bunch of angry mob on your back (As long as you can sustain the damage, don't overdo it), get to a bottleneck, cast firewall and just watch your foes burn and get bit by your death aura while you stand and laugh (Although undead aren't affected by the aura, firewall is good enough). In addition, gained the Phantasmal killer spell (Taken at level 8), and started insta-killing stuff. While you don't have Illusion focus, the DC is good enough on its own at low levels.
Grabbed wraith form at level 12 and had twice the fun; 20% addition miss chance to the 50% from displacement = almost never being hit. Gained the spell Necrotic Ray, does decent damage for only 10 spell points and lower target by one level. Gained the spell Circle of Death (at level 11), started using it often. Continued the technique of firewall. Continued insta-killing with Phantasmal killer. From this point onwards I almost never die, if I do die then it's 95% of the times after the rest of the party is dead, or it's entirely my fault. 5% the self heals are just not enough, but that again it's my fault.
At level 13 gained the spell Finger of Death, Otto's sphere of dancing, mass hold person - utilized those very often. Sphere of dancing is great as a trap, though not very useful at the mid-level quests. Mass hold person is just great, freezes targets + Makes them take more damage, very helpful for a party. FoD... No need to explain anything there. Continued methods from lower levels, stopped using Phantasmal killer as the save was too low and I had no Heighten Spell at that point (A mistake, should've been taken at level 5.)
At level 18 exchanged a few feats; dropped metnal toughness, had greater spell focus enchant and necro as well as greater spell pen, heighten, quicken, empower. Took lich form, spells DCs are high enough to almost never fail (38 necro and 37 enchantment, no special int boosting gear.), by that point I dropped firewall (Re-speced as ice and some electric)
Using almost nothing but crowd control (Mass hold mosnter, dancing sphere, web [!!]) and insta kills (Finger of death, circle of death, wail of the banshee [!!!])
When I want to do DPS (Often only against bosses and some hard red names) I use Eldar's electric surge and Niac's biting cold, stacking them over and over with Empower. Doing very decent damage, especialy if a sorcerer is around to boost it.
Also reached 410HP unbuffed (Lich form + 45 HP Green steel conc-opp item + Toughness [Bracelet of Thaarak] + Greater False life)
Will take Maximize spell at level 20, expected DCs: Necro 41 Enchantment 40 (With no super hard to get gear), spell pen: 29 (With no super hard to get gear). HP: hoping to reach 480.
Summery: Firewall up to level 18 or so. Drop it from that point.
Remember that you are NOT the party nuker, you are there to focus on specific targets, insta kill them, and save the day with your crowd control. Don't feel bad sitting back in easy fights, save your spell points for when you'll need to save everyone's butts, trust me they will thank you.
My two cents!