Lawn of the Dead Strat, or "Cute Singing Sunflower!"

I did the final wave of zombies two or three times before I found a great strat on Wowhead.

Early on, keep the plant Sunflowers on cooldown till you have about 3 made. By then you should be dealing with about 3 Zombies. Rocknuts are your friend, build a few to hold them off.

Next is planting Spitters to kill off those 3 Zombies once you have enough Sunpower.

Start planting more Sunflowers, the key to winning is making two full rows of Sunflowers. Balance making those with a full row of Spitters and one row of Freezya's. You can even toss in a row of Rocknuts to be safe. Then try and stockpile Sunpower for Pumpkin Bombs.

4-5 Pumpkin bombs should win the game

My board looked like this:

Sunflower : Sunflower : Spitter : Freeza : Rocknot : Rocknot  :  :
Sunflower : Sunflower : Spitter : Freeza : Rocknot : Rocknot  :  :
Sunflower : Sunflower : Spitter : Freeza : Rocknot : Rocknot  :  : Pumpkin Bombs | Badguy
Sunflower : Sunflower : Spitter : Freeza : Rocknot : Rocknot  :  :
Sunflower : Sunflower : Spitter : Freeza : Rocknot : Rocknot  :  :

And if anything died, I'd replace it.  I got a double row of Rocknots because the Pumpkin Bombs have a long cooldown so why not, right?

Because I used the Pumpkin Bombs near the badguy, he died pretty fast.  :)