http://i.imgur.com/ZAPxYDq.jpg?1 You want to make sure you're charging into that corner, holding triangle as soon as you can and aiming for that position. If you fire a fireball and miss, adjust slightly above / below and spam them. Be a little patient when you fire them as they do take quite a while to reach the button. I reached this position through aiming down with triangle and using savestates to get into a position which hit the button.