You have reached a stage of burning barrel study that you can create a minor illusion of a burning barrel that is strong enough to move and set on fire with magic. Since it is not touch any more, reduce damage to 2d6, but still have a 15 foot square (cone or diamond, 9x9 in any shape).
rolling range from top of the stairs probably does not count if there is no roof
I think the range should be 30 feet, or 15 feet from the bottom of a stair case. Because that is about as far as you can see when looking down a stair well.