Killer combo early on till freedom islands

[Rinah]

Actually you can only get a maximum 6 Skeleton Archers per coffins. The rest is more likely from gravedigging.

You can buy undead troops from the furious paladin after you kill him. And you can also buy Skeleton Archers from the necropolis in Marshan swamp before conquering it. That should be enough for a killing undead army.

As far as I know, there is no way to get necromancers early on, however.