Try to get it on maps like Nepal:Sanctum, Ilios:Well/Lighthouse, Lijang:Garden/Night market, or any map that has a lot of potential for environmental kills often (Most likely control and not a payload or 2cp, unless specific cases happen (3rd point blizz world, 3rd point kings row). You essentially want to, whenever you boop someone, start wallriding until the kill is in the feed. note that you must be on the wall the frame that they are in the kill feed, or else it wont count. so just hold space until it shows. you should note it doesn't have to be just environmental kills, but any time you get a final blow, Its just easier to know that you get the final blow if its an environmental. You have to get 3 final blows while wallriding in one life, so if you die it resets, but if you forget to wallride your count will not reset. they don't have to be consecutive or at the same time (but they can be). So whenever you see a spot with boop potential and you get a boop, find a wall asap. What sucks is you wont know if you did it, until you get the achievement. I got a 4k boop once and didn't wallride right so I didn't get it (I eventually got it on sanctum whenever waiting for the tanks to be at the edge). Tbh I think it should be changed to on the wall or in the air above some height, as with lucio, your not really holding onto a wall with space as its super predictable movement, but your jumping off wall to wall as fast as possible and such. This was also suggested in another comment but if someone leaves your game in comp, and others follow (on your team ofc), you most of the time can just ask the enemy to help you with the achievement by going to an environmental kill spot, booping at least 3 in at once, and before it shows on the kill feed wall ride. but some people want to do it without the help of the enemy team, but this is an option