Pretty much the same as my guide for the "Rinsed 'Em achievement
As before the best way that I found to boost kills is to go in solo Graveyard Ops. The more human players there are the more people there are to share kills between.
Choose the level "Time Park" and set to easy difficulty.
Spawn in as a Scientist with the healing station and not the beam. For Aggro reasons I used the Toxic Scientist but any will work.
Once spawn in spawn in 3 Captain Deadeye using the 3 graves (Variant here doesn't really matter)
Spawn Graveyard"A" in Zegypt.
While it counts in the first wave go through the teleporter back to the central hub and sit on just the other side of the Zegypt teleporter on the Central hub side. There should be a slight blur caused by the warpgate. This is the prime place for aggroing mobs as they go through.
This is where the toxic scientist comes into his own as with his proximity damage he will aggro a lot more.
For some reason the majority of mobs will come through the central hub to get to the garden even if there is a direct teleporter from where they spawn.
You should only need to go back to Zegypt if your Captains experience any issues although to be honest the only times I had to go through was if they were dealing with a boss.
Place your healing station when you see a large wave getting close or in need and just sit by it and kill all plants that come close.
I was averaging 160-170 kills per Op and each op was taking about 18 minutes. In one Op I peaked at 236 kills but it will all depend on what "bonus" waves you get
As you are doing Graveyard Op's as with the Garden Op's there is a potential you can get the Super Boss Waves for the "Not the Boss Of Me"" Achievement
It is worth noting the most basic weed plant and the big bosses are all worth one kill each so choose the groups of lighter enemies over the occasional plant hero.