So, first of all, obviously you want to get the Death of Orion as soon as possible, if you don't know how to get it, there are guides available for it. You should be able to do this in a solo match by around round 10. Once you get it, I recommend getting enough to pack-a-punch it because this weapon does have a high chance to make crawlers if not hit accurately so upgrading it will help reduce that chance.
Once it is upgraded to Serqet's Kiss, start training zombies down in the pack-a-punch room. Once you have a horde of around 20 zombies, you're going to want to either run under the stairs (if there is fire going on you can also run through the busted doors in PaP room) and get through the arch so the zombies get super tight. Try to hit the highest cluster of zombies without charging your Serqet's Kiss.
It took me about two or three attempts and I got the achievement fairly easily. Try to do it on as low a round as possible to prevent crawlers because if they don't die, it won't count. Best of luck!