Using Lord Relias, I tried to use burning hands against an earth elemental. Hovering the mouse over the earth elemental showed the crossed out staff cursor, and the tip "Need to target enemy unit." I didn't try it to see if it would work as I knew the fight would be lost if it messed up (I lost all my army but Relias, but I won -- first time I've realised what a difference a mega-buffed army can make! 
). If I get it again when I can afford some health, I'll test it and update the thread.
