Thanks for the replies. Yeah, I had forgotten about the "one per zone" rule, but remember it now that it has been brought up.
@Shadow, have you had problems with your opponent destroying your Poison Gas Cloud quickly? With the ability to take whatever you need on any given turn it seems like grabbing any Ethereal spell would kill this almost instantly due to its low life. Even the Wizard's innate ranged strike could probably kill it in one shot. It seems to me like the most you would probably get out of the spell is a single turn of damage (maybe two) before it got taken out.
As I said, I haven't tried it yet, but it seems like it just wouldn't last long before it gets taken out.