Don't underestimate the garlic. Plant sunflowers on your top and bottom rows leaving two spaces on the right. Then place two pieces of garlic in the remaining spaces. That will make all your zombies move in one row, then you can really amp up your fire power and get those zombies before they eat your brains!
That's actually a strategy I've yet to try... sounds like a it'll work real well. For one reason or another, I've never made use of the garlic much (guess I did underestimate them!)
If you do that, you've got more chance of getting more zombies with a cherry bomb, which is one of the achievements. I've only got about three because I've not got that far into the game yet. The garlic was a tactic I used in the PC version.