Plants vs. Zombies Garden Warfare

I've found that using the Achievement app's new Snap feature can help out here. When you're getting close to finishing, simply Snap the app and then just kill Plants. If you need that particular class of Plant, then the progress bar will increase, and if you don't need that class of Plant then it will obviously stay the same.
This will then let you know which type of Plant(s) you need to focus on to finish the achievement.

I've noticed this can glitch out sometimes, if the bar doesn't increase then quit the Achievement app and then relaunch it, and it should fix itself.

I have to agree wing samsaBEAR that a great method of tracking kills towards the end is by snapping the achievements app and watching the progress bar increase when you kill a specified plant. I just wanted to go a little more in depth with what the percentages mean against amount of plants:

For this achievement you need to destroy 100 of each of the 4 plant types. That means that the achievement is broken down into quarters - 25% for Peashooter, 25% Sunflower, 25% for Chomper and 25% for Cactus.

Breaking this down on the next level, the 100 kills by 20% makes it 4 kills per 1%. Therefore when you are getting close to the end and only need one type, you can gauge roughly how many more kills you will need.

For example, I am currently at 96% on this achievement. I know I only need to kill Cacti. Therefor by applying the above I know I need no more than 16 more kills to finish the achievement.

In regards to actually killing, I say focus on completing the game modes and pretty much all other achievements. You should get this achievement around level 50 and if not, you can use the achievement snap method to identify your required targets. There always seems to be a good mix of players in any game mode, but I do find Cacti to be more prevalent in Gardens and Graveyards.

Long answer I know, but I hope it can help some. I know I was getting frustrated with this achievement as it entered 90% and up and once I understood the maths behind it I realised why it was taking a while to unlock. Stick with it!

Edit - Put 5 kills instead of 4 by mistake. Also changed 20 kills to 16. This was pointed out by both Shadow 00 Fox HamsterLV. Thanks for the heads up.