Step one: Go to the Multiplayer Portal in your hub world and select Private Play.

Step two: Select the game mode "Turf Takeover".

Step three:
For Plants: Choose the map called "Loggy Acres".
For Zombies: Choose the map called "Tourist Trap Island".

Step four: Set the difficulty to "CRRRRRAAAZY!!".

Step five: Click to team setup and be sure both teams have 12 members.

For Plants: Make sure only Captain Deadbeard is selected (sniper AI can't aim) and for plants select only sunflower so they perma heal you while not stealing your kills.
For Zombies: Choose only Cactus as plants and Scientist for zombies (same reasons).

Reminder: Recommended to select both Acorn and Sunflower while playing as Acorn,
Space Cadet and Scientist while playing as Space Cadet.

Step six: Be sure all of the Crazy Options are disabled so you can gain experience.

Step one: It is recommended to select the upgrade "Leveling Up" since it will make the grinding a little faster.

Step two: Select your character and rush to the payload (it's right outside at the enemy plants or zombies base).

Step three: Sit on the payload and wait for the game to begin.

Step four: Shoot down the enemies until the game ends.

Step five: Repeat. (watching something on a different monitor is recommended for your mental health)

From most chill to most frustrating (personal experience):

Plants:

S: Acorn (pretty tanky and does area damage), Pea Shooter (a lot of damage overall)
A: Chomper (tanky, use only the ranged attack since you need to be on the payload to gain the most experience)
Rose (using goatify and jinx gives a lot of experience, making the process way faster)
B: Snapdragon, Sunflower, Nightcap, Citron, Cactus
C: Kernel Corn, Wildflower

Zombies:

S: Engineer (put a turret in front of the payload for additional support and unlimited ammo, most vanquishes I got with a character while grinding)
A: Space Cadet (Station Inflation on top of payload and shoot) All-Star (use the dummy since blocking also gives experience) 80's Action Hero (a lot of damage overall)
B: Electric Slide, Scientist, Captain Deadbeard, Imp, Foot Soldier, Super Brainz
C: TV Head, Wizard (just don't bother you only need 20 anyways)

1) Why the payload mission? The main purpose of this grinding is that how the game rewards you for vanquishing an enemy while on an objective, even though they are bots. So you can also do the other turf takeover mode where you capture or defend an objective base. The reason why I think the enemy payload missions are the best for grinding is because if you are not letting the enemy to push the payload down you'll constantly spawn kill your enemies. That's not the case in objective missions since you have to wait for them to move to you from their base to the objective which lowers your experience gain per hour.

2) Experience numbers: You get 25 experience for vanquishing an enemy hero, 100 if they are a real player. Now why would you waste your time with bots then? Because First: bots are easy to deal with, Second: you'll most likely won't be able to get kills on multiplayer that much because they will get stolen, enemy will shoot you down, they will run away, and so on. Too much time wasted. How about garden/graveyard Ops? Ops do give a good amount of experience but not good and fast enough compared to this strategy. This saves the most time out of the things you can do in this game to grind as far as I know.

3) Keeping yourself safe while grinding: For plants side even though Deadbeards have terrible aim, they will be a threat if they get too close to you because they will deal a good amount of damage with their shotguns at close range. If they are coming to you in barrels the explosion will most likely one shot you down. My best tip for this would be jumping on top of the payload and snipe Deadbeards off before they can get too close. Now mind you, they will get close to you at some point because of their numbers, but you should most of the time be safe up on the car because-
One: Sunflowers are constantly over-healing you, giving you protection against shotgun pellets. Two: The explosion won't reach you if you stay on top of the enemy payload. You might get targeted more by shotguns if you stay on top. So if you feel like you are getting low on health I'd suggest you to step away to the back of the payload so you give your Sunflowers more opportunity to heal you faster (since they are AI they get a harder time to keep their focus on you if you stay a bit far from them)

This area will be filled with more tips and trick as long as there are questions that needs to be answered. So please type them down and I'll gladly assist you on your journey.

You won't have fun you just have OCD and will never sleep properly without getting this achievement