The Zombies are utterly worthless. You have to attack Otomo across the river to spawn them and the Zombies can neither run, nor will survive a walk to the bottom part of the map in the first place. And dividing your forces to attack the wolf in an already fortified position with towers covering each other is suicide.
However Necroman lacks the stamina to run to the bottom part and summoning SWs, or vice versa with summoning SW and then having no stamina to fortify your position at the bottom/keep the aggression up.
Swap the starting zombies out for SWs, which would fix the aforementioned problems and would also make the mission indirectly easier. Hell, even peasants would do. But zombies are utterly worthless.