The quickest way I've found to do this is go into Giddy Park on PRIVATE WITH AI settings. To set this, go to the Globe at either Zombies or Plants home base, and you'll be able to set that setting. As Imp, go to the other side of Giddy Park, where you will see Wildflowers (they look like Dandelions) being shot from the air-ramp into the area. Keep shooting them when they land (they're invincible when flying into Giddy Park). Use your mech too, for quicker kills. 200 Kills later, and this achievement will pop.
This is pretty quick. Maybe 15-20 minutes.
A good idea is to have a friend with you, and then he can have you constantly being healed by the scientist to work towards the achievement below:
Just switch characters with each other when you're done, so you both get the achievements.