I did this ridiculously easily. I was shocked how easy it was. First things first, do not go online and rely on randoms. Most people have little to no idea how to play this game. They will just be a liability. Either get three competent friends, or use AI allies. Here's how I did it:
Aqua Centre.
Choose the garden on the right from spawn (the hard-top, enclosed area, not the grassy area with the fountain).
Be a sunflower (preferably a metal petal). It helps if this variant is maxed out with full abilities.
AI companions - Citron, Corn, Toxic pea.
Resist the temptation to get up high. You need to be on the ground, mobile, and able to react quickly.
Plant all of the pots, and between each wave replace/heal any that need it. Right at the back, plant a scaredy-shroom. A healing flower close to the garden is advisable, but just the one. Most of your pots need to be offensive. Think about the placement of your pots (e.g. do not put a scaredy-shroom right next to the garden, in harm's way, or a bonk choy far away from the action).
Heal. Heal. Heal. Keep your allies alive. You will need them. Let them do the majority of the shooting.
If an ally does go down, do not put yourself at risk. If the healer dies, then so will the rest. Stay alive and on your feet.
If you find yourself in the position of there being just you and one other ally, simply stay close to that ally and constantly heal them. Use them as cover. Let them take the hits instead of you.
And that's it. I coasted through it. Good luck all. 🙂
