Diamond Rank, 29:38 first try.
Spam sun/puff early game, focusing on their upgrades, and use Wall/Hypno when necessary, Ice as last resort. Midgame pivot to fume in C3-6 for AoE. Late game spam Wall/Hypno/Gravebuster/Ice as instas as much as possible. Gravebuster is notably immune to damage here, so you're safe to plant him, but I'd personally wait since Fume can clear hordes. Only put Scaredy on the roof and only upgrade if no other worthwhile options. Don't use or upgrade Doom Shroom until you have absolutely no other choice besides Floor Sun.
I'd love to see an expert mode that makes the early game harder, since setting up is the hardest part.