What is the best way to get gems/diamonds?

Basically what someone else said. Do your quests every day, and watch advertisements. You also get gems for climbing the ladder in Ranked mode.

Hi

Try completing the quests that Dave and Zomboss give you to complete. Also hero quests are worth giving a go. If you don't want to try one in a multiplayer battle for fear of losing stars, try having a casual battle or a single player fight. If this helps, please grant XP and I'll return the favour!

Hope this helps, William! 😉😉