I was having a huge amount of trouble with this one until someone have me a tip. This is what you need:
2 rows of sunflowers
3 rows of tentacles.
Start planting sunflowers early on, and plant tentacles as needed. Save two rows for all of your sunflowers! Get them up ASAP. One tentacle should have no problem taking down 2 zombies, so don't worry about it. Once all your sunflowers are up, start planting tentacle vines. I ended up with three rows, but you can probably get by with two. With two rows of sunflowers you get so much sunlight that you actually start looking for things to spend it on.
After you have the sunflowers and tentacles up, make sure you have enough sunlight for a pumpkin bomb, and plant rocks as needed. Only use the pumpkin bomb on the big purple guys (and anything bigger than that). The tentacles will take care of anything else. If your pumpkin bomb is on cooldown and you have sunlight to pare, plant a few rocks.
Once I used this strategy this level was very easy. You can use this same strategy on the final level as well. Don't bother with spitters or freezies, just sunflowers and tentacles.