There are no potions, spells, area-of-effect attacks, reach, smoke bombs, or climbing you can do (unfinished game). Your options are to either be a legendary ambusher, be a vampire, or have legendary fighting skills and kill one necromancer at a time, pretty much rushing to read the slab somewhere in the upper levels.
When you fight multiple opponents, especially if you "charge", the group fighting you can get attacks described as "from behind" and "from the side", and those are devastating even if you are covered in armor, with legendary armor user, shield user, and dodge skills.
Move around a lot and try to kite the zombie mob so you only hit one at a time. If necessary, dump the bodies outside line of sight so necromancers won't raise them. Maybe you can light campfires on them to burn them away.
Also, there is a bug where if you leave the tower and come back, new necromancers will be loaded in the tower. So you can't lead the zombies far away to kill them and whittle their number down, then come back.
Most weapons are pretty good against zombies, just try not to cut off their limbs.
I guess one advantage you sort of have is that necromancers feel pain (I thiiiiink). You can get an easy bone-breaking hit on one, then deal with the mob (or their other wizard buddies) while they're unconscious.