This may seem a bit daunting at first, however it is not as bad as it seems. Your first playthrough will get you 500-750, depending on how many times you die.
Assuming you collected all the scrolls throughout your first playthrough, you will then unlock "Pro Mode" which is just a mode where you start with all skills. There is a seperate achievement for completing this mode, meaning you will have to play the
whole of story mode again. Once complete, you will then have around 1250-1500 kills (overall).
Now here is were a good spot for kills comes in. At the very start of Episode 10, you will have to clear out a bunch of enemies and then use onion to put the trophies under the arches. However you
do not want to do this.
Instead clear out all the enemies and you will notice there is an infinite spawn of Spicy Chillis. You are going to want to switch character to the Onion and climb up on to the pipe (above the collectible on this level) and then sit on top of the rocks beside it. You can then just put your controller down, preferably rubber band the right analog stick so it doesn't turn off and let Broccoli Joe kill them as they spawn. You will get around 400-600 per hour, however I would advise checking back every 15 minutes to make sure he hasn't died. If you come back and he is on low health just switch to him and walk into the green health circles.