Farming skyscraper (greatcloud endboss) takes 4 to 6 seconds with a polymorph mage (airbender or a proper genie). it grants 2400 gold and time enough to cast all spells you want to boost. a miscast won't use mana so it's failproof provided you don't spawn creatures close to the boss (cc triggers big spell that may eventually kill your polymorpher).
Diadem recipe:*R*/MMM/M*M
 "Royal Heroes"){kind=icon}
 "Battle Conquest"){kind=icon}