This level confused us a little bit. While we got through it pretty fine, even on our first try, we decided to play it again the next day to make sure that our strategy still held up. While we faced a Gargantuar upon our first try, there was none to be found the next day. We are not quite sure if it spawns randomly, but keep in mind that you might or might not face one when playing through level 22 of the Jurassic Marsh. That being said, you should stick to the following strategy either way.
Day 22 challenges you to defeat all zombies with 3000 sun at your disposal. That means that you cannot collect any further sunlight during the level. We quickly made up our mind as to which plants to choose. We recommend equipping yourself with Winter Melons, Bowling Bulbs, Chard Guards, Primal Peashooters, and Perfume-Shrooms. You will face the regular group of zombies including the Jurassic Bully as well as one T-Rex and various Raptors, and Pterodactyls. You can find our set-up on the screenshot below.
With this set-up, you should have fairly few problems. We reserved more than 500 sun for usage during the level depending on situational difficulties. Two situations that we had slight problems with were the top lane in the beginning when the Raptor kept kicking the Jurassic Bullies to the front and the second lane from the bottom where the Pterodactyls. Either use a Perfume-Shroom or defend yourself with Chard Guards. Use your three green leaves on the Winter Melons as they will cause the most damage on the entire map. Towards the end of the level, you can always destroy the plants that are not needed anymore to position more plants in problematic lanes.