Tactics against Castle.

That's why you need to hoard skeletons!! Expert Wizardry, Skeleton Converter and all that! You can develop Diplomacy and recruit all the first- or second-level creatures, and then—one breath and they are already skeletons)))) And since building angels is a strain in terms of gems, you can capture some mines, if of course you manage to reach them first)))). With some kind of Logistician. And when there are many skeletons, it takes longer to chop them down! As for the battle... well, you can hit the angels immediately with something—for example, Terror Knights, for double damage) and to make sure they don't resurrect each other or other units.
Does your friend split his angels? What is his tactic? Or do you just need a GENERAL TACTIC?

And playing as Necromancers is actually great against humans! There are plenty of people to drink blood from; these aren't elementals or golems, everyone is susceptible here!