It's not really a build, I just spec into most of the passives except pyromance, ice armor, and winter stillness. Virulence, unsullied victim and overwhelming force are the bread and butter.
I play necro with 3 skills. Of course this is one of the class I'm using for doing promotions over and over. Walking bomb is looking like the champion for my 3rd skill. However, because I promote this class I sometime use blizzard instead, but the mana cost isn't always worth using this skill and it attracts archers. Whereas you can use walking bomb on the one archer that is attacking you and wait for the pinata to release the sweets. I also use a fire staff and would recommend using one if you have a good one. The reason for using a fire staff is that virulence will pass on the burning affect to near by enemies which is pure extra damage. Pure damage! That is what you are going for and also passing on status effects to near by enemies.
If you are pro, I am not, you could go for mind blast as the third skill. I've tried it, horror on a group run in hope your team hasn't done enough damage to release fear and use mind blast to do nightmare damage. I didn't find it fun...
Maybe I was overstating thing by saying your doing it wrong, but that's because I was thinking about a necro for non routine level playing with people who have 50+ promotions (*SMH*). I used to use keeper for promotions, and still think keeper is better but just not as fun imo.