Plants vs. Zombies Garden Warfare 2: Deluxe Edition

This is what I found works best, of course you want to begin this grind with the highest level/promoted plant/zombie that you currently have to get that time down. For me, I was lucky enough to get the BBQ Corn Kernel Corn variant on my very first pack opening in the game and found his loadout to match my playstyle perfectly, so I grinded for this achievement with him.

What I did was load up a private match in the multiplayer portal and changed the settings to what follows:
Gamemode: Herbal Assault
Map: Zomburbia
Difficulty: Easy (Of course you can change this if you so wish.

Team Setup:
Plants: Plants attack
Zombies: Z7 Invasion

Make sure you have XP x 2.0 for your character!
Each imp has very low HP, and gives you 50xp each kill. 100xp if they're in a mech.
I was getting roughly 40 kills per match, so this was a guaranteed (minimum) 2000xp (this isn't even including how much extra for mech kills) for around 10 minutes of gameplay (7 to get to the final party crash). Of course this time can be brought down if you wish, or increased by stalling capturing the point for as long as you wish.

I hope this assisted!