Lol ok that's weird but I always get this spell here! At least 4 times when I didn't forget and never something else. Well I saw in another post someone mentioning that this spell was one of the non random item of the game, so if it's not with this quest item then it's with something else I guess.
Are you using the 1.7 version? That could depend of the version.
About tactics in this fight I forgot mention points I'm using too:
- Take care don't kill enemy stacks but only reduce them.
- Necromancers aren't the priority at the very beginning of the game.
- Before starting finish killing stacks lowering Necromancers very low is the point to do.
- Mass magic chain is nice for first round if you have it.
EDIT: Well for sure it's quite tougher in impossible mode but other fights too!
EDIT2: I bet you are wrong about the necromancer spell, it is always in the book, I just start a new game and did the quest and get it one more time and the text quote the spell when you get it. It's a book about Necromancer, there's logic to get that spell in particular. All of that isn't a proof but a high suspicion that this isn't a random spell, very low chance that I always get it if it's random. Perhaps it's dependent of the game version.
EDIT3: I see the confusion, it's not the quest reward but in the item quest, you have to use it to get the spell.