I've translated and organized the method used in this video since it's in russian, where basically you play normally as survivor and at the end you set up a second survivor player by using splitscreen through console commands. I've also made a guide for TRUCK STOP using this method, both are also on AStats. I've tried multiple methods and this is the only one that ended up working.

1) "map l4d_river03_port versus" in the console to load the map in versus mode, you should start as survivor.

2) Play normally as survivor and get everyone on the bridge so you can press the button for the final part where someone needs to sacrifice themselves. I suggest binding these two commands:

"sv_cheats 1; warp_all_survivors_here; sv_cheats 0"

"sv_cheats 1; noclip; sv_cheats 0"

They're mostly for traversing the map quicker and teleporting the bots faster to the bridge, so you can press the button faster as they take some time to actually go on the bridge.

3) After pressing the button, run these commands in the console: "ss_connect" to initiate the splitscreen, and "cmd2 jointeam 2" to add the second player to the survivor team. Without the second player the match just ends if you switch teams.

4) After setting up the second player, jump off the bridge with your survivor, so it triggers the achievement requirement, and press M to switch to the infected team.

5) After switching teams you can wait to become a tank or just run the command "sv_cheats 1; z_spawn tank; sv_cheats 0" to immediately become a tank, additionally you can run the commands "kick tank", "kick (1)tank" and "kick (2)tank" to remove the other tanks so that you'll have an easier time downing the survivor that left the bridge.

6) It should pop off after you down them, worked flawlessly for me.

The video also shows how to do everything if you're having issues. Feel free to comment or add me if you need help with this or other achievements.

Credits to MrTolya28, I was having issues with this achievement before discovering their video.

Thanks for taking your time to read my guide and I hope it helps!