What are you reanimating in a Dwemer ruin? Falmer?
Personally, I went up the Atronach side for Conjuration as I preferred the speed and reliability of using an Atronach rather than having to locate and use a corpse, but if you want to stick with the Necromancer roleplay, I support that too. It's not "wrong", just different than what I did, and I have no problems with it.
Along with my Atronachs and Follower, I used Lightning Magic. It honestly doesn't seem to work very well against Spheres, but it was, at the time, the only Destruction Magic that I had added perks into.
You might consider carrying a Staff with you as backup since at level 12, your magicka pool is pretty low. Just be aware that using a Conjuration/Summon staff will kill your already-reanimated Zombie. Until you get the Twin Souls perk, you can only have one Summon/Zombie at a time regardless of how it was created.
While making the fight a bit more cumbersome and risky (in that you may kill your follower), you could also buy a Dog. You can use both a Follower and a Dog. Add your Zombie to this mix and you can go 4vs1 on most any enemy. Dogs can be purchased at the Markarth Stable.
What are your current perks? What type of armor are you wearing? We could suggest more if we had a bit more info.
Do you dual cast your Zombie?