Hearts appear during the game depending on the size of your combos, the bigger the combo, the more hearts you'll get. You must collect ten hearts in a single level. Just try to create large combos with as many blocks as you can as if you would while trying to get the Shaman, Warrior and Sun King achievements (I nearly got 40 blocks while getting this).
You also need to be careful not to miss a heart although it is quite tricky getting three hearts at a time especially if they are far apart from each other. You can finish the level with a big combo and still collect hearts and it will unlock as it did for me.