I don't know the legality of defending yourself from across a border, but if someone was shooting at me, I'd sure as hell be defending myself by any means necessary.
If you fired back, you would see it all over the news. By the time it made it to MSNBC, Rachel Madcow would be telling her few veiwers about the racist homophobe with a fascination for guns and fatty food who was so intolerant towards these undocumented democrats, that you would not even let them shoot at you without showing pure right wing hate.