Make it so that if the undead army fails to kill their target in 75 rounds, then they all dogpile on the one who cast the spell in the first place
That would make quite a few people a little more wary of casting it on an SC god, I'd say...But, that seems overly complicated, so I just vote 3. I hate to see a unit, even a very tough unit, fight the good fight, only to be betrayed by the game engine, of all things.
Another possible solution, though, would be to strengthen the *quality* of the undead, based on the unit they're attacking's kills, instead of the numbers. So that at 100 kills or so, you get 100 longdead, 250 you get 100 shadows, 500 you get 100 ghosts, 1000 you get 100 banes, all the way up to Tartarians or whatever-keeping in mind that I really didn't research the exact quality of one undead over another, except in the most general of ways, but I think the point's clear enough for comfort.