I've been testing some different setups with necromancer, and I'm weighing in favor of not using poison proc corpse explosions anymore, because they don't do enough damage if you can get the cast speed of bone spirit fireball down to below .5 secs. I changed my mind about corpse explosion after I got some better gear and got a bit higher cast speed and damage.
I can crit for 12-15k with fireball and launch 2 per sec. End game for necro is about fireball DPS while using fears to lock down the largest threats. I save howling terror as an "oh shit!" button. Dark Elf's leech ability can top necro off very quickly, so between that and howling terror you got a few ways to bail out. You can also use the opportunity to howling terror => run if you end up getting like 3 chromatic champs on the same screen, which does happen sometimes. Putting 20 points in invoke death is a good idea if you plan on doing chains because you won't have to worry about micro managing your pet. The pet is going to get around 50% of all the damage, so consider it a shield.
As far as the best way to chain, it took me lots and lots of deaths to get the feel for the limits of necromancer, and even then I still get killed occasionally. But I typically pull 3 mobs and try to position where fireball will hit 2 out of 3, with the 3rd being a place filler to allow me to continue the chain. If I get a champ on the screen, fear it if its off CD if not then use bond of death. Then I blast all the left over regular mobs before I kill the champ, and then hit pull as I'm about to land the killing blow on the champ, then pull 2 more times to get 3 mobs on the screen and repeat. Another important thing, after you hit pull, hit tab to see if the mob you just pulled is a champ, if so then there is no need to pull another and risk a triple champ gang rape, if worst comes to worst, fear one, and bond of death the other, or use your "oh shit!" buttons. The last time I got pwned it was by 2 frenzied + chromatic + disruption aura gorillas. That's a deadly combo for a necromancer, because your heals won't leech worth a damn off chromes, and they hit like a bus. I'd suggest howling terror, then RUN if you come across 2 worst case scenario champ mobs on the screen.
If you plan on chaining with necromancer, its a good idea to bring a shield. Pretty sure this is the best shield:
I found a dagger earlier called Chromaspike, and it seems like a solid choice for a 1h caster weapon that would be good to combine with either the above shield or the Holy Grail.
