Board Thread:Game Discussion/@comment-26227413-20150320213141/@comment-9605025-20150322022416

You have to change your "deck" before fighting him. In the upper left of your screen should be a button with 9 small squares arranged into a larger square. That button is a short-cut to your deck where you can edit the cards used to fight waves and bosses.

I am assuming you are relatively new to the game? Then you should probably just hit the "recommend" button and choose the "balanced" option. This will arrange your deck according to some algorithm that the game uses. Note that you have 4 different decks and each one must be edited separately.