This is the easiest way i have found,
Play a game of Gardens & Graveyards
Choose the Pea Shooter. Make sure you have the chili bean bomb and hyper abilities unlocked
The plants are allowed to freely roam the map just before the game starts, while the zombies are locked behind doors.
Use this opportunity to get as close as you can to the zombie spawn point,
As soon as the match officially starts, press RB to activate hyper.
Run all the way to the zombies and drop your chili bean.
You need to have played on each map a few times to know where the spawns are.
You should get first strike consistently this way, just be sure to hide behind something after dropping the chili bean as you will likely have a ton of zombies shooting you, and you can not get first blood if you are first blood.
