One of the two achievements in Overwatch that are specific to Lucio is called "The Floor is Lava", which requires you to get three killing blows whilst wall riding. Does that mean you simply need to be wall riding at the moment of the kill, or when firing the killing projectile? Maybe even both?
This Video might help you.
You must be riding a wall while killing three enemies without dying in between the kills. To clarify, you need to be wall riding when the elimination happens and you receive the notification on your screen. What the guy does, and seams to be easier is to push them off the map and just ride till they hit the bottom.