I got this one after completing adventure mode and playing the first level again (where crazy dave selects 3 plants you have to take with you).
The only plants you will need are coffee beans and hidey shrooms (think thats what they are called?). Dont bother with the mushrooms that create sun, we are just gunna use the sun that falls from the sky!
Collect all the sun you can, and place hidey shrooms at the back only when you see zombies spawning in that lane. Obviosuly use coffee beans the wake them up.
When it starts to get a bit more hectic, you need to have at the very least 2 hidey shrooms in each lane. and a 3rd in the lanes where the majority of zombies spawn in the final waves.
Your final set up will look something like this, depening on where the majority of zombies spawn for you:
hs hs hs
hs hs hs
hs = hideyshroom.
Honestly, getting the sun producing shrooms cost me to much time and I found it easier without. Hope you do too =)