I must have been in a hurry when I read this before, sorry. Anyway…
It sounds like your necro is your first hero to reach this point in the game? If so, you are not doing as bad as you think you are. The first playthrough is obnoxiously hard in the last two chapters, and necro is one of the harder classes to do first. It was my first hero, and I remember really struggling there at the end of the story
Can you survive the first (non critical) hit from the first enemy in the dungeon? If not, you have to find a way to improve your armor number and/or your starting mana. Dragonguard breastplate might help with starting armor and Dragonguard helm plus bone rings are the gear to seek to max your starting mana. Other helpful gear will be Savior’s shield and Dwarven necklace. For weapons, I like ones that raise your power via matching. Like Royal Longsword and Guard’s Broadsword. Basically, its not so much about the level of your gear, as it is about what your gear does. This is a handy guide for where to find specific gear: [GUIDE] Dungeon Gear Drops by Chapter
Once you’ve worked out surviving that first non critical hit… you want to shoot for a single color spell load out. That is tough to do on first hero without farming for spells. My favorite spell set for my necro is: Channel Ice, Cold Snap, Ray of Frost and Frost bolt. Legion of Doom could replace Frost Bolt, to make better use of your ultimate.
What you are aiming for is to end every turn with either a stun, a barrier, or a chip kill (kill from matching gems). Try not to kill with a spell if you can help it, because you hope to have full mana for the next enemy.
There is still going to be a luck factor, and some reloading to get good starting boards. Good luck! Edit: blurred some details that might be considered TMI/spoilerish.