I have been trying and trying to get this achievement for days and I got so frustrated, but I think I finally found a system that works. I hope it helps you too.
1) Get the magnet first!
Then get the star and the threepeater (the pea shooter that shoots three peas at a time). It’s important to get rid of the magnet first because it interferes with so many of the zombies and can result in waste of sun. Once you have gotten rid of the magnet you can use the digger zombie, ladder zombie, bucket head zombie, and the football zombie without any fear wasting sun. The reason for getting the star and the threepeater next is just because their ammo interferes in other lanes and sometimes you can forget that they are there.
2) Go big or go home.
If you have the sun, spend it on strong zombies! It’s cheaper to get the job right with a strong zombie then to use a bunch of cheaper zombies and waste the sun.
3) Watch your sun!
Check your amount of sun before you start every level. If you're low on sun then you need to aim for a sunflower first before you can further strategize. I know this might sound like a no-brainer, but it’s gotten me a bunch of times.
4) Leave enough sun for the next level.
You need at least 50 sun to summon an imp in the next level; if you start a level with less, it's automatic game over.
The pole-lunging zombie can go over potato bombs and chompers. This is something that took me way too long to figure out. Just remember that it can’t really jump over the squash plant because it will just squash you after (which is fun to watch, though).
Even though you can jump over these obstacles, sometimes it’s best to sacrifice if you think that line is a more than one zombie kind of thing. If you're going to sacrifice try using an imp, just remember that the imp can’t get far without dying, so only use imps if he can make it to the squash plant in question.
7) Protect Disco!
Use a cone head zombie in front of the dancing zombie, he’s fragile and worth a lot of sun. Just remember that he does a fast dance move before he summons his back up dancers, so you're going to want the protective cone head zombie to be a couple squares ahead of where you're going to put your disco zombie.
8) Beware Streak Eight!
Level 8 is where I hit game over most of the time; I just simply ran out of sun. So be extra careful on this level!
9) Love the Digger Zombie!
Don’t underestimate the digger zombie, he is extremely helpful in the right situations. Just watch out for bombs and remember that you need to use an imp to collect the brain after.
10) Better Together!
The cone head zombie and the pole-vaulting zombie make a great duo. If you place them both on the same line the pole-vaulting zombie doubles a shield while it attacks the next plant. This helps get rid of two plants at once while also furthering the distance of the cone-head zombie. This is a good move to use at the beginning of each level when you're desperate to get more sun.
Thank you for reading my article! If you think I left something out, let me know and I’ll add it to the list.