If you really want to go with undead army, then the Eviln spell is a must. Eviln transforms a corpse into an "Eviln" which resurrects undead troops, and can be used by Warrior hero as well, even through the mana deficiency. That can be achieved by getting the Mana Accelerator skill for your pet dragon, kill enemies, spawn mana accelerators, and finally spawn evilns. If you're out of corpses or otherwise can't use them, level 1 Call of Nature or something similar will net you plenty of corpses, because the stacks you spawn will be small and will die easily.
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