Hi, thank you WakyTaoaster ^^. I have thought about that solution(enemy.ImagePointX + slash.ImagePointX)*0.5), but for some big/giant boss if I slash to the leg under the knee, the blood might appear somewhere above the knee.
I am thinking about the rectangle collision polygon that contains FX Slash sprite and Enemy sprite. If I can get the position of these vertexes of the rectangle, I can find the intersection of the edges and find out the overlapping area. I don't know for sure if it's possible but will try ^^.