Stones are in isometric view, that is, we see their front, side and top edges. From this, collision will not happen when the boundaries intersect exactly, but with a little overlapping.

Objects have a curved shape. To simplify the calculations, I can consider they are ellipses. Even easier to regard them as rectangles.

When the main character is below the stone, the stone's z-index must be higher than that of the character. The stone hides the top of the character, as a roof. I added effect like the stone is pushed up by the minion.

When the character is above the stone, the stone's z-index must be smaller than that of the character. Then it will be, as it were standing on top of the stone. I added effect like the stone is pressed down by the minion.