I had recently wanted to 100% the game, but playing organically I got to about 300K kills. I didn't want to keep playing solo for hours to get to 900K.
I figured out how to use Cheat Engine 7.2 and edit my kills to be over 1 mil, instantly got me the achievement. Here is how if you also wish to do the same.

Step 1> Download Cheat Engine (7.2 is the version I used)
Step 2> Run Over 9000 Zombies and enter a single player game and get a kill (e.g. 2)
Step 3> Pause the game, run Cheat Engine
Step 4> In Cheat Engine (File > Open Process > Over9000Zombies.exe)
Step 5> Where it says "Value" type in the number of kills you have (e.g. 2), and click "First Scan"
Step 6> Get another kill (e.g. 3) (This will change your value for total kills, so we can isolate it in the long list of code)
Step 7> (Scan Type > Increased Value by) if you are following the examples, your original kill count was 2, and now it is 3. so where it says Value, type 1 (3-2=1), Click "Next Scan"
Step 8> Repeat steps 6 & 7 until there is only 1 thing on the list.
Step 9> Right Click the Value of it > Change value of selected address > Type 1000000 > Enter
Step 10> Go back into the game, you should have gotten the achievement.

Please remember this is only if you want to cheat the achievement.
I've worked out how to make you invincible, unlimited scrap and grenades etc, use it for fun, not to take advantage.