I stream every monday on Xbox One to unlock my unicorn chomper, Im quickly hitting half way there (gaining now 3-4 ranks a day about 10-12 a week with only a couple hours of play). One thing I can suggest is dont bother spending coins on anything but the 10k frozen sticker pack. They give me now after a lot purchases/ranks of only buying that 2-5 skip stars per pack, every now and then i get a 10x skip star pack. I am now in a sweet zone where I can complete, 4-5 challenges a character per G&G and skip the other 10. I earn back more than I spend each time i play and now have 250 skip stars and 400k of coins to buy more packs.
When you first start purchasing the pack you'll end up with a lot character decorative pieces but eventually (usually around rank 35-50 if you only spend your earnings on that you will get mostly skip stars). play garden ops to complete plants challenges quickly and easy, saving your skip stars for the stupid challenges. (you'll learn the stupid ones quickly).
Shamless self promotion if you wanna come out and watch on mondays; https://www.twitch.tv/thecoopcouple