In early waves, total sun given is limited. You need to wait for Sun dropping down from the sky which is not enough for most cases. Therefore you need to place Sun producing Plants like Sunflower, Twin Sunflower and Power Flower. According to your knowledge, the earlier you put Sunflowers, the more Sun you will get throughout the entire round. Otherwise if you put them last, you will only manage to get a few Sun and the battle ends. However, they are not attacking Plants. How are you going to live until you have gathered sufficient Sun to defend?
Plants vs Zombies Adventures how to survive early wave - Aim corners and multiple paths
To survive early wave, apart from putting Sunflowers, you need to put cheap shooting Plant. You need to save Sun until you manage to put the total number of Sunflowers you plan to put in this round. You have to only put a few shooting Plants so their position is critical for your survival. A good strategy is to put them with the maximum attack range covering the corner. Putting this way will make sure you can hit the Zombies long enough to finish them off before they approach your Plants. The second rule is to cover multiple Path if possible, so that instead of putting 2 Plants on each Path, you put only one covering 2 Paths for temporary offense measure. Look at the image above. The Zombies need to walk through 6 square areas on the left before reaching the Peashooter. Imagine if it is only a straight Path. If that is the case, Peashooter can only shoot for 3 square areas. Also, you can see that this position is ideal since it also cover 5 square areas of the right Path and the beauty of it is that it is not place beside that Path so it is impossible for the Zombies to eat that Peashooter! How wonderful it is!