Why not a spell like "poisom arrow" ?

[Oversway]

I think its a cool idea, too.

so that a certain air/nature nation with lotsa bows wouldn't get it too easily

Man is mostly a mid tier nation, from what I've read here. I don't think this would throw game balance off so badly. Other nation exist with really easy access to cheap arrows. Machaka for example. I'd also say that a poison arrow type spell would have the same problem as fire arrows - once storm and arrow fend are out, its not nearly as big of a threat.