Overwhelmed by corpses, didn't you?
It happens to the best of us.
I have now found a way to efficiently raid the necromancers' tower with low amount of risks.
First you might want to be a legendary
stalker ambusher.
Standing in a building with the far right and left of you being walls, starting sneaking (Cltr+s) and press both left and right.
WOKKA WOKKA WOKKA
You'll flail around like an idiot while being invisible. Just keep holding the two buttons while you're sneaking until you notice your speed change. Check your level, and continue on.
You might also want to stop when you get hungry, thirsty, or drowsy and take care of that before continuing.
In no time you'll become a legendary ambusher
And then you'll want to be a legendary thrower
Get load of rocks and throw them around, most preferably to your own tile for time efficiency. Do this until you become legendary.
Now you'll want to raid the tower, also remember that having followers won't help anything at all and soon they'll become undead as well.
You will want to sneak before approaching the tower, when the reach the place you might want to stay away at least one tile from the enemies just to be safe. Just keep throwing rocks at the necromancers until their brain cracks.
If you can't reach more necromancers and there are lots of animated corpses, then you should reveal yourself and lure them away from the tower as much as possible, then run away from them and sneak again, come back to the tower, rinse, repeat until all undead and necromancers are cleared out of the tower and then you can read slabs without any deterrence.
Oh right! You need to be a novice reader, too.