This level is very similar to previous levels, with the three common types of zombies and Poncho Zombie. The main focus is to kill Poncho Zombie as soon as possible, and as efficiently as possible. Using minecarts in conjunction with powerful plants can help. Buckethead Cowboy makes one appearance here. Do the same thing as dealing with Poncho Zombie.
The small number next to the zombie icon means that zombie will always appear in that lane, with 1 being the topmost lane and 5 being the bottommost one.
Do what you need to do first like planting sun-producing plants, delay the times for gathering sun, and other stuff.
As you can see, Pea Pod is a stronger version of Repeater and Gatling Pea from Plants vs. Zombies, since it can shot five peas at once if you maximize its heads, so plant a five-headed Pea Pod on the first minecart from the left of the lawn. It will make it the most useful plant in Wild West.
While you do that, do not forget to plant Wall-nuts and other plants, such as Snapdragons to help you.
If you can, you can plant one column of maxed-out Pea Pods, but it will take a lot of sun.