Plant one column of Kernel-pults and as much as the powerful attacking plants as you can. You need to plant one column of Wall-nuts too along doing so. Plant even more Spring Beans to instant-kill barrels, Buckethead Pirates or Imp Pirate Zombies if needed.
Meanwhile, instant-kill zombies with the Lava Guava to get rid of crowded groups of zombies.
Don't forget to use Plant Food on Spring Bean or Kernel-pult to get rid of many zombies at once.