The Floor is Lava achievement for Lucio is meant to demonstrate one of Lucio's key wall riding mechanics. A player can enter a wall ride while facing away from the wall. Its both useful as an escape mechanism as well as applying more damage. There is a section of Lijiang Tower which is particularly useful to demonstrate this. I made this short video to go over the mechanic as I have not managed to get the achievement yet. I also think that the way the star map looks while you are riding on the wall is particularly stellar looking. This is a second place to try it in addition to the large boop and charge in on Ilios:Well.
*edit: there is an error in the video. The reason why you fall isn't because of the pillars on the top route, its because you fall after 3 seconds. You can make to to the places to stop aim shoot down then continue to the next stop point which is how I did the achievement.
you can also do it on the pillar in the middle of garden on Lijang. You can also rotate up there for the whole game and never get noticed .... (I even told my team but had to switch character in the end to get him down there)
Paired myself together with a Zarya to get it. Pulled them of the ledge and I booped them while wall riding. I think they need to adjust the achievement as it was okayish to achieve before the damage nerf to Lucio, back in beta.
Edit: So i did it today on this map using this method!!!
I had one push off outside then 2 kills above using this route, stopping on the little stop spots to shoot then continuing the route. I don't have a video unfortunately... but this map still is probably the best to do this achievement i think. Almost all the walls can be ridden and you can alternate between above and outside. So stoked to have completed it today by shooting people with this method. Again, sorry I don't have a video.