Refer to Plant vs Zombies: Survival: Fog (Hard) Tips
1. To fight with zomboni, winter melon is not good enough. Need to use pea shooter with fire. Place it at the right side (where the roof is flat).
2. Do not use kernel pult (not much point). Use cabbage pult at first, later change all to winter melon.
3. Place cactus at the right side of the roof (where the root is flat), otherwise, it can't shoot properly. And also you need to plant one cactus per lane in order to shoot down balloon zombies properly. Thus, it would be a better idea to use blover altogether (more effective).