Why do people care about the plausibility of this or any game? It's a game. If you really want, I'm sure it wouldn't be hard to come up with an explanation, but who cares?
Easily got to 30 minutes on survival with the following spells fully upgraded (til I got bored):
Angel of Death
Skeleton
Locusts
Confusion
Polymorphism
Earthquake
@lionheart87, what was your starting set up, how many survivors did you have, what day did you get beat it on and how did you keep your happiness so high?