Plant two columns of Sunflowers while planting Potato Mines to kill the zombies at the start of the level. Then, start planting one column of Kernel-pults and one or two columns of Cabbage-pults. Plant extra Flower Pots when needed.
Then, plant two columns of Wall-nuts to defend yourself from dropping zombies at the final wave and other zombies coming from the right hand side of the board.
Use instant-kills to kill big groups of zombies or ladders when placed on your Wall-nuts.
If necessary, plant an extra Flower Pot and an instant-kill plant on it to kill extra zombies.