Merge monsters together to make new ones. But what's the point? You can buy the best one from the shop! Mine rocks to get coins. You already have infinite, though... Better rocks give you more coins. Thanks to @alphabetica !!! Sorry there are some glitches but im trying to fix 'em!!
All credits to @alphebitica , I just added the infinite money and changed the code so that the rocks and monsters are highest level from the beginning!!!