This achievement can be done alone, provided you have access to two PC's. For example a desktop pc and a laptop. It goes like this:
1. Launch steam in offline mode on the second computer. Launch the game like you normally would on the host computer, which is the one getting the achievement. This is all under the same Steam account.
2. On the host computer set steam launch options to '+director_allow_infected_bots 0'.
3. On the host computer type in console "sv_lan 1" and launch the map with "map c6m1_riverbank versus"
4. After the host computer has done loading, you will be able to join with the second computer. It will show in the main menu on the bottom center with "Steam group server:1" with your username scrolling. Join the host computer from there.
5. After the second computer has joined, put both on the right teams. Host computer plays as Infect. Second computer is playing as Survivors.
6. Die to the survivors until you get a charger. Do not spawn yet and bring the charger to the wedding area. This way you can go through unopened doors.
7. Start playing your way through to the wedding area together with AI survivors on the second computer. Clear out the area.
8. From the host computer, kick all AI players, with kick command.
9. Position the player in front of the chairs and from the host computer use the Charger to charge.
This can easily be done if you put the laptop close to the pc, so that you can see what' s going on on both screens and coordinate everything.