Use Doom-shrooms and Ice-shrooms when zombies either get close to your lawn or when too many zombies spawn at one time (for example, big waves of zombies or final wave). Try not to plant Doom-shrooms too close to the house, as the craters formed after they explode can stop you from planting Scaredy-shrooms you might need later on.
There are thirteen graves. If they are not taken out, they can spawn thirteen extra zombies on the final wave.
Use Grave Busters on graves for more tiles to plant and to avoid 13 zombies at the final wave.
On the final wave, there is a perfect opportunity to earn the achievement 20 Below Zero. Be sure to save an Ice-shroom for the wave if the achievement has yet to be obtained.