We've added a help indicator on how to optimize your deck in 0.09 but for now here's my tips:
1) Have around 10 cards in your deck, 10 is the magic number. You can have 1 or 2 less or more depending on your preference.
2) Use BIRD cards when your opponent is using Ultra Sense so you aren't at an automatic disadvantage.
3) Save your high powered cards for when you have Ultra Sense so you can guarantee high damage
4) Use consumables, stock up by playing the easier enemies.
5) If the bosses are too hard, just level up. The bosses don't scale in difficulty.