• Launching map

Copypaste this to the console: ss_map l4d_garage01_alleys versus
This will launch the "Crash Course" campaign in "Versus" splitscreen mode.

If there is Survivor on the left screen, then repeat the command until there is Infected on the left.
Don't kick bots, they will defend our AFK target.

• Target location

To position our target (this is the second player that we cannot control) where we need it, we will need to use the console command to teleport the survivors to where our crosshair is pointed.

Copypaste this to the console including quotes:
bind g "sv_cheats 1; warp_all_survivors_here; sv_cheats 0"

Thanks to this, if you press G, then all survivors will be teleported to where your crosshair is looking at. If you want to use another key, write it instead of "g". This bind does not lock achievements obtaining, and it will also be useful for you to get a bunch of other achievements alone.
All you have to do is move our AFK survivor slightly, position him right in front of the railing on the helicopter side. In my case it was Bill, teleport the survivors to where he is in my screenshot.

• How to properly aim the jump

Before trying right away, you can first look in the spawn mode from where you will need to jump from. Behind the spawn point there is a tall enough building that you have to climb on by two drainpipes. Find out in advance where these pipes are located.

Remember the place on the roof you need to jump from in advance:
- This is the second tile counting from the top of the roof;
- This is the lower third of the tile, not the middle, not the seam, but the lower part;

From this point (and while lining up to our AFK target), it will be necessary to aim directly at the sun. If for some reason that doesn't work, then aim slightly below the sun.
- If you are not a Hunter - spawn and let the bots kill you.
- If you are a Hunter - climb on the roof, look where the target is (to not miss), then spawn and follow the above instructions.
- If you miss, leap away so you don't get killed by the bots and climb onto the roof by the pipes for another attempt.

I tested this tactic several times, works flawlessly.
At the top of the screenshot, you can see that exactly 25 damage was dealt.