Sunny Days

First Level
This achievement can be done easily by collecting suns in the first level. First, plant a Peashooter, and the wait for suns to start falling. Collect them all until you have 8000 sun before planting the second Peashooter, and the achievement is unlocked. This works because the first zombie will not come until the second Peashooter is planted.

This does not work on the first level if you have already beaten the game once.

Normal Strategy
The strategy is described like this: Play a day/pool level that has 3 flags (level 1-7, 3-2, etc.). Keep building Sunflowers in rows 1, 3 and 5 and put 2 Garlics at the front of each row. Then in the other rows, put a few offensive plants such as the Peashooter, Snow Pea, Repeater, (Gatling Pea should not be used because of its high cost), Cabbage-pult, Kernel-pult and Melon-pult etc. If replaying level 1-1, only one Repeater or two Peashooters are necessary. In about column 7 (6 if in the water), plant a Wall-nut/Tall-nut with Pumpkins protecting them. Finally, fill out the remaining spaces behind the defensive plant with Sunflowers. You can use Sun-shrooms and/or Twin Sunflowers.

Alternatively, you can also plant Spikeweed and Spikerock instead of offensive plants (if playing level 1-1 or another simple level). This is especially effective if planted behind defensive plants.

Last Wave Strategy
During the final wave, kill all zombies except about 1-3 zombies. Dig up all plants (except for Sunflowers!) and plant Sunflowers in their place. Block the zombie(s) with a Wall-nut/Tall-nut, then collect Sun until you get more than 8000 Sun, then kill the zombies with an instant kill (make sure that after you use the instant kill you have 8000 Sun or more!).

Filling the Pool Strategy
If you have unlocked it, play the Zombie Nimble Zombie Quick Minigame. Start by planting sunflower on the first column of all the grass rows. The first zombie you will have to use a squash on it, then when the second one comes, plant a Lilly Pad. Then place a Cattail on it. Do this for both water rows Cattails will attack all zombies in all rows, even backwards. They are as fast as repeaters also, and having them in a fast paced level makes them better. Now continue planting sunflowers on the second column. Slowly upgrade them all (8) to twin sunflowers. Also plant wallnuts on the 3rd or 4th last water rows to slow down dolphin riders. Plant 6 (3 per row) Cattails in total. If you do it correctly, you should get it during or slightly after the third wave.

Easy Way
Play the minigame Zombiquarium but don't buy the trophy. Just keep collecting sun. This does not work for iPhone.

Survival: Endless
Build a strategy that lasts long enough to collect Sun. This is very easy if you have good timing on Digger Zombies and Giga-Imps.

Survival: Day and Survival: Day (Hard)
Plant a column of Repeaters at the back, plant a Tall-nut in column 7, plant Spikeweeds in front of the Tall-nut. Fill out the remaining spaces with Sunflowers, and upgrade them as fast as you can, as well as upgrade the Repeaters and the Spikeweeds.

You could also use that same strategy, except replace Repeaters with Chompers and Tall-nuts with Pumpkin. Place Chompers in the 3rd and 4th row, or beyond. Place Pumpkins on top of the Chompers. Put some Spikeweeds and/or Spikerocks in front of the Chompers, and fill the rest of the empty spaces with sunflowers.

Survival: Roof (Hard) and Survival: Roof
Just plant Twin Sunflowers at the end of each row and play as normal. Just try to be lucky to do not find any bungees zombies or catapults ones. Seriously, can be problematic if you find one.

Seeing Stars
Fill the specified spaces with Starfruits but leaving one. Then keep collect sun until you have enough, and simply plant the final Starfruit.

Zombie Nimble Zombie Quick
Simply applies the method "Last Wave Strategy" above, although in this level, you can even max out at 9990 a lot faster. This is also a good candidate as it is a long level, being 4 flags.

Zombiquarium
Simply continue in the minigame without collecting the trophy. This is probably one of the easiest and fastest ways to get this achievement.Make sure you have lots of Snorkel Zombies.

NOTE: this will not work for the iPod Touch/ iPhone version.