You can do this solo and much quicker by doing it in scavenge. I followed this video and got it on the second try, took about 10 minutes:
In case the video gets removed, here are the steps from its description:
1. From the Steam Workshop subscribe to Improved Bots (Advanced) mod. (THIS IS IMPORTANT!)
2. Start the game and enable Developer Console in the Options Menu.
3. Type "~" (left key of #1) in the game menu and type in ss_map c6m1_riverbank teamscavenge
4. Repeat step 3 until you spawn as a zombie.
5. If you spawn as a zombie but not as a Charger, immediately get killed to have another chance to spawn as Charger.
6. If you fail, repeat steps 3 - 5 until you spawn as a Charger
7. Once you spawn as a Charger, wait for any of the survivors to reach the chairs and charge accordingly. Be patient.
8. Pray you hit 8 chairs as you charge a survivor. Repeat steps 3-7 until you succeed.
9. Achievement!
Steps 1 - 3 is necessary so you can do it in single player.
(all credit to jelome1989)