Board Thread:Questions and Answers/@comment-24.85.91.54-20130504042114/@comment-5706972-20130507150452

I think you are asking 2 different questions: 1. Element boost (for your own deck) and 2. Matching element (how your element fairs vs. opponent's element)

1. Element boost is 5% - That is to say, if you are Fire: every fire card in your deck (front and back) gets a 5% boost to atk/def. More info can be found here: Battling

2. Each element is supposed to have an 'opposite' element (like: rock, paper, scissors). I attempted to find the discussion on this, but I couldn't find it. There is supposed to be a boost when you attack your 'opposite' element.