I just got this achievement playing alone with 3 bots. I recommend following the path shown in the videos above. I started on easy with all three bots, they are quite useful to attract hunters, jockeys and other infected that could attack you instead. I tried to go alone after killing all the AI but always got pounced/charged/etc. Since the AI always stays a lot behind, I switched to expert difficulty just before taking the final path on the right that leads to the helicopter, so they got quickly killed and I finished the level without having to wait for them. Maybe at the beginning of the level you could injure them a bit to ensure a faster death, but I didn't need to do it.