Mahou Tsukai Kurohime

In a land where magicians cast spells on bullets in order to perform magic, like summoning powerful monsters. There was once a witch so powerful that she challenged the gods and was cursed for it. 10 years before the current time of the story, that witch challenged and defeated another magician, and in doing so saved a small boy, who instantly fell in love with her and was inspired to use guns as his "Sword of Justice". That boy was called Zero. When he was ready, he set out on a quest and found that woman who saved his life... or did he? She is not the same person that he idealized; she is selfish, rude, arrogant... and quite a bit shorter. But at the times when she feels Zero's love and self sacrifice for her, she can bring forth incredible power. So the two set out together to find a cure and the reason why the gods only bothered to curse her.