Split Pea Zombie Soup can be very difficult to get, as only players will count, not Spawnable Zombies. The best place to get it is in Gardens & Graveyards due to the length of the game, but it is easier if the plants are bad at defending, as the game will last longer. Try to attack weakened zombies with headshots to use the least ammo and maximize amount of damage you get out of each use of the ability. You can also do this in Suburbination, as multiple players will try to get to a base, so use this to your advantage.