Sunny Days

Interestingly enough, the maximum amount of Sun you can get is 9990. (Through cheating it is possible to get 9999 sun but it is normally impossible to get sun in increments other than 5 or 10.)

Survival: Endless Strategy
&nbsp If you have a good build and a small amount of luck, there won't be any need for replacing expensive plants for a while and you can easily save enough sun with four Twin Sunflowers. A four-cob build is recommended because, unlike the two and no-cob builds it requires minimal amounts of instant kills or maintenance, and the six and eight-plus builds require split-second timing to avoid damage to plants.

Standard Level Strategy
A strategy for this achievement is to use lots of Wall-nuts and Tall-nuts to subdue the Zombies while you collect as much sun as you can. You should definitely do it on a Day level to have the extra sun falling from the sky. Pick a somewhat long level so as to prolong your chances of collecting sun, but make sure it isn't a level where zombies need to be your first priority. A good level to do this on is the Mini-game Seeing Stars, as it can last as long as you want it to and the zombies are not particularly tough. Just build up enough defenses to easily deal with the zombies, plant a row of Twin Sunflowers in back, and wait to plant the last Starfruit until you have enough sun.