The zombie horde becomes somewhat difficult to deal as the level progresses, mainly containing many Cowboy Zombies to act as shields for more protective zombies like Poncho Zombie. Area-of-effect and defensive plants are recommended here. Additionally, since Prospector Zombie appears, using Split Pea or an ice plant is also advised.
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 get ready. Start by planting Twin Sunflowers on the first column. Then, plant a Repeater on the minecart when you get enough sun.
The Poncho Zombie is a special zombie because there is a chance he will wear a metal grate, which is as tough as a bucket. So be careful of it. To stall him at the high threat, use the Chili Bean now. It will instantly kill a zombie, regardless of its toughness. This is an excellent choice for players who don't want to buy premium plants.
When the final wave comes, do the same.
Take advantage of the Chili Bean. It will greatly help you replace the Squash, a premium plant.
Small tips: The Magnet-shroom can attract the Poncho Zombies' metal grates. You may choose it to make the level easier, If you have the plant already.