Just join every Friday the event 15 rounds challenge, try get round 10, or at least 14.
And earn a lot of strange peaches. There will some suprises from those fruits, that you can increase the pet's amity.

Other ways:
- Waste some stars to buy some bouquets of flowers from Juna in Welling Village. (Don't waste stars too much!!!)
- Use red thread, a reward from the quest in China, but be careful it breaks easily.
- Buy some amity jellies from Item Mall.