Okay so this guide is to help those with a 'Death of Orion' which must first be obtained to obtain this achievement.
Pack-A-Punching the wonder weapon to a 'Serket's Kiss' voided the achievement and I could not unlock it with this weapon. I also could not seem to kill 9 zombies with a single UNCHARGED shot no matter if the round was low or high but the first time I tried this with insta-kill on I got it.
The accompanying video was created by myself and shows me using the 'Death of Orion' (note that it IS NOT Pack-A-Punched) and training zombies in the starting arena till I finally got the achievement. You won't see the achievement pop as the video was made using the in-game recording and please note that the action unlocking the achievement was the shot at 0:20, but did not unlock until a few seconds later which is coincidentally after I shot a second time.
This took me 4-5 rounds of training in the spawn arena and trying to kill 9 with a single uncharged shot as the elemental zombies and gladiators kept making this difficult but like I previously mentioned. The insta-kill powerup made this work instantly. Hope this helps: