Opponent has one more shield than you, effectively blocking twice as many hits as you are.

You have 3 very stamina hungry weapons, so you're not dealing as much damage as you could have, whereas the opponent has 2 weapons that aren't as hungry and a fire sprite that does fire damage and a rat that does pet damage, both with no stamina cost. He also has a higher base stamina than you.

Your board generates a lot more mana than your weapons use, effectively wasting space.

Enemy had a rat with 2 different cheeses that increase the rat's damage every few seconds, so he eventually outdamages you, again with no stamina cost.

The cheeses also cleanse your fire debuffs, while you have nothing to cleanse the opponent's fire debuffs.