Technique No.1) Buy 500-gems chests from the Treasure Cave. Edit
Save your gems as much as possible. Only do what gives you the most xp with the least hearts to keep replenishing your hearts as you level up. Multiplayer doesn't cost any heart and will give coins and xp along with PVP Season Rewards.
Technique No.2) Spend your gems on Heart replenishments Edit
After 2-3 months, if you play every day, when you start to miss out on hearts but have an excess of gems, replenish your hearts and keep getting more of the cakes or ingredients or whatever you need to keep building up the creatures you have.
Technique No.3) Do easy battles with good drops Edit
Once you find a battle that has the stuff to upgrade your team, do it over and over and figure out how to outsmart the AI with the right combination of creatures and moves. Since you will eventually almost never lose, that battle will also be a constant source of xp to level up and battle more.