How do I create a lot of skeletons?

He attacked different enemies, including Gnolls, which have 6 HP, just like skeletons - but still, he only managed to deal 1/5 of their health in the best-case scenario :(

That can't be! The Necromancy expertise gives a 30% bonus to skeletons, and that's definitely not 1/5. I won't calculate the fractional value mathematically, but I know it's approximately 1.5/5. And if there's an amplifier, it could be 2/5, plus artifacts or even the Grail. And if you're talking about the start of the game, do you really want to resurrect 100 skeletons from 30 Gnolls that are guarding a couple of chests???