Easiest method? Solo Op!
Step 1: To get this you need to plant a plant in a pot. Make sure you have something plantable in your inventory by earning yourself 2,500 coins and buying the smallest pack available to get some starter plants.
Step 2: Go to the RV and start a Solo Op on any level. Once you're in, you can plant AI partners.
These don't count. Once you plant the garden on an indicator position pots will show up around the level, usually 3 within eye sight of the garden you planted. These are not the best options as the zombies may have a hard time even approaching them before your team or you kills them. Go out searching quickly for a more distant pot. Approach!
Step 3: Press
on it! Pick a plant! Preferably a tactically positioned plant based on its description, but it doesn't really matter. Just don't pick a plant that says it doesn't fight! Pea Shooter will have the best luck. with the splash damage. The next step is to leave it alone! The achievement will ping off eventually for you as long as zombies spawn near your new little buddy.