Strategy for every wave: just fire the eight cob cannons at the incoming horde (not too fast ideally, you should always have one ready to fire just in case), and replace pumpkins as needed. The two gold magnet-shrooms will auto-collect all the coins that drop, which is a ton in the later stages.
Pumpkins protect most plants and that's pretty much it for necessities. Cattails are handy for balloon zombies and stragglers. Ice melons slow every row down while the umbrella leafs/gloomshrooms protect the back plants. The gloomshrooms (8-way shooter purple mushrooms) in the water kill anyone in the pool, and damage anyone in the adjacent lawn rows. You'll definitely need ice shrooms and bombs.You can go pretty much indefinitely with a setup like this (or similar): The fume shrooms should take care of everything else.
Doing enough damage to zombonis and the giant zombies. The greatest problems you will have after level 30 will be using more sun than you can produce. I reserve a couple of empty squares just because it gets too expensive to replace things. you'll also notice that I don't fill every single square. By offsetting them a couple squares, I have a little more time. You'll notice that I use a V formation because my outer rows always get hammered by the giant zombies.