Please, to all players, do not put empty/uncomplete decks as defense deck it's just not fun to kill a player with no cards and you may lose rating points. So please check your decks.
About prismatic gems they are useless, full levelled dual gems and all the mana cost/damage/range upgrades, are the best way. JUst try a grade 8 dual gem with the max upgrades you can get...It's madness