Now keep in mind what sora said , and remember to save fairies ( especially of platinum - sometimes gold - and up ) to use with platinum armors that give xp , obtained from monday dailies or various events. 1 platinum armor gives 2k xp to a unit. combine that with a fairy of the same rarity as the unit you want to combine , and you would get 4k xp. combine with 3 fairies , and it would give 8k xp. 8k xp is enough to insta lvl a unit to lvl 40 or so , but you should generally use that when your units are already cc-ed and past lvl 50 because combining then takes a crapload of gold