If you had like an area effect spell, then you could turn the necrowagon around. eg. If you had an archmage, cast Blizzard on the wagon area to do damage to the wagons and any skeletons that may be revived, but then you may run out of mana. I once used some tauren and shaman combo (it was late in the game) and pulverize can help (60 dmg shockwaves) but then you would be using heavy armor on these guys, so the necro attacks would hurt a little more, but not life threatening if you have high dr and some guys. But go for lower lvl removers (Dryads, Shaman, etc.)