What is the best way to farm coins?

You could collect all the 54 golden gnomes and then solve the secret puzzle in the secret gnome room. If you have no idea about what I'm talking, just search the internet for help. Doing this will net you 600k points. You also might consider talking with the clam in the suers and collect the 50 snow globes. Each globe gives you 500 coins and once you got them all, you'll earn another 100k. If you've got all these, go into Solo Play, and play a round of Herbal Assault or Gardens and Graveyards with both teams being composed only out of Sunflowers and Scientist. They will constantly heal and revive their comrades, so you can get a ton of vanquishes, which equal coins and Xp. Xp can be very useful too, since promoting a character gives you 20k coins instantly.

I hope this helps