Wall Nut and Bonk Choy
These two plants make a fantastic combination and defeat so many zombies really easily.
Basically you just need to plant a bonk choy directly behind a wall nut (or similar blocker such as a tall-nut, infi-nut or endurian) and it should be an effective smasher of numerous zombies.
You can also team this combo with spikerock or spikeweed in front for even deadlier line-up.
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