First off, The Bowie Knife buyzone is on the wall when you open the door behind the room with the power switch. The Bowie Knife costs 3000 points, and it stays with you permantly.
Depending on when you get the Bowie Knife, this can either be an easy or hard achievement to get. If you get this in the early rounds (around 5 or 6), you'll have more than enough time to get this achievement. If you get it later on (Around 9 or 10), you're gonna have some problems.
Just keep in mind that the Bowie Knife loses it's ability to insta-kill at round 12.
The way I did this is to wait for the zombies and knife them as they come out me. This isn't actually a good strategy unless you haver Juggernaut and can survive if a horde rushes you.
It's also beneficial to have a decent weapon if it looks like the zombies will overwhelm you. Some reccomendations are the FG42, the Trench Gun (both for 1500 points), the Thompson (1200 points), or, if you must, The MP40 or Type 100 (Both priced at 1000 points.)
A final note: If you get Insta-Kill in the later rounds (Around the time the dogs mix with the zombies), go on a knifing spree! Just again, be aware of the possible zombie/dog rush and also consider the 30 seconds you have.
With that, good luck!