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The thing to keep in mind also, is that it is one way or another an illusion, regardles of how it's presented. As such if say some one threw up the illusion of a wall and hid behind it. Said illusion would not protect them from any fire directed their way (intentionally or otherwise) save to make it from a normal shot to 'wild fire', as anyone failing the save would not be able to see what they are shooting at.

Of course generally the idea in such situations is to have the illusion up in place before hand.