I'm on my nightmare run too. The build that I found working for me is relying heavily on companions. I have Solas with Rejuvenating Barrier, Dispel + whole Rift mage tree and Vivienne with the whole Inferno tree + Knight-Enchanter basics.
Then my build is at level 18:
Necromancer (9) - Horror+, Walking Bomb+, Haste, all the passives
Storm (5) - the left side: Chain Lightning, Stormbringer, Gathering Storm, Static Cage+
Inferno (5) - Immolate+, Flashpoint, Clean Burn, Pyromancer
Winter (2) - Fade Step, Winter Stillness
I take all the CD reducing passives to be able to cast Walking Bomb as many times as possible. (Plus I have a mana reducing staff with Fade-Touched Ring Velvet and the extra mana for the dying mobs.)
1. Horror+
2. Walking Bomb+
3. Chain Lightning
4. Static Cage+
5. Fade Step
6. Immolate+
7. ???
8. Mark of the Rift
This leaves me with one empty slot, I currently put Haste there for fun, as I've never tried it before.
I put down Static Cage, then Fire Mine with Vivienne in the middle, an early detonated Walking Bomb, and a Fire Wall for good measure. I also love to use Varric's Elemental Mines on top of this (has to be my favorite skill in the game!). Prepare for lots of bams and bums. 
For me Horror has 3 uses:
- when an enemy is coming dangerously close (Horror before he can touch me!)
- when there's too many enemy (Horror on the melee enemies while I focus on the archers)
- to deal with the archers (when I drop the Static Cage on them I cast Horror to make them try run outside
)
I've yet to kill my first dragon so I don't know if this is viable there too...
I have a question though. I saw a lot of mentioning of the ice necro on this forum but I don't see how that could work. Anybody knowledgeable about this?
One thing I heard about that the class seems to have going for it was a combination of simulacrum and a heal-on-hit effect that would make the necromancer essentially unkillable: turning into a spirit upon death then healing upon dealing damage and being resurrected when simulacrum ends as your health is no longer 0. Haven't tried this myself though so can't vouch for it.
I tested it because I was extremely curious and it works!