It is advisable to go for this achievement last.
The quickest method for getting the 500k kills is to utilize the Chat Commands for Twitch. Primarily, the 'killallzombies' command. This will kill all the zombies on the screen but cannot be used again for another few minutes. To bypass this issue, just restart the round. You do not have to be streaming to use the commands. As long as you link the game to your Twitch channel, you can use the commands.
To get this achievement as fast as possible, do the following:
1) Go to Options and turn on Chat Commands AND link your Twitch Channel.
2) Launch a Survival game.
3) Pick any weapon as it does not matter.
4) In your Twitch Chat, type in 'killallzombies'.
5) Press Start and Restart the round.
6) Repeat 3-6.
NOTE 1: This works for all of the weapon kills too. On average, you should be getting anywhere between 50-100 kills per rotation. You can get 5k kills in about 3-6 minutes. So, to get 500k kills it will take anywhere between 5-10 Hours.
NOTE 2: If using this method, make sure you press the 'cog/gear' icon under Twitch Chat to bring up the options. Click 'Popout' and the Chat will now be in another window. This way you can watch a video while spamming the commands.
NOTE 3: A full list of commands can be found here,
http://kilallzombies.wikia.com/wiki/Streaming_interactions