Range at the back
This is the first basic piece of advice you should heed in any Plants VS Zombies game. Take your ranged attackers, and stick them at the back of the field. Or the left side, in this case.
Zombies will approach from the right, and ranged attacks should be on the far left, away from danger but still able to fight the zombie hoards.
If you think this is obvious, then you've never seen someone put a peashooter on the far right of a stage. It doesn't end well.
First line of defence
So, what units should go on the right side? Well, I don't think any unit should go all the way to the right (no one in life should swing all the way to the right), but there are certainly units who should be in the mid-field.
The Wall-Nuts are a great example. Wall-Nuts will block the advance of enemy zombies, giving you more time to damage their health.
Put Peashooters and other ranged enemies behind the Wall-Nuts to take down the zombies from relative safety.
The first big issue you'll come across in the game will inevitably be zombies floating on balloons - those cheeky buggers.
To take down these enemies, you will initially need a lob-type unit. The early units are a cabbage and a corn cob - I can't remember their in-game names, but one is a cabbage, the other is a corn cob.
Put one or two of these units behind a Wall-Nut and you should be able to bring down the floating zombies so Peashooters can finish them off. That is until you get the Snapdragon…
Death by dragon
The Snapdragon is OP. It's just the best unit I've seen by far, and destroys all enemies so simply.
The Snapdragon fires flames over the course of three lanes - the ones it is in and the two adjacent - and shoots those flames across two columns. Sounds good? It gets better.
Give the Snapdragon a taco upgrade, and suddenly it flies, attacking over three columns, including floating enemies. Two Snapdragon units placed on the second and third rows, upgraded, will basically end every game by themselves. A full row of dragons can take out almost any enemies.