Thank you for pointing this out. I think you are right that the difficulty increase between Battles 3 and 4 is currently too much.
The intention was for Battles 4–6 to introduce the second tier of cards and represent a more difficult stage of the game. However, giving the opponent an entire second-tier deck immediately creates too large of an advantage, especially before the player has enough strategic options to compensate for the difference. At that point, the result can feel more dependent on luck than on the player’s decisions.
I will review the balance and deck composition of Battles 4 to 12 with a more gradual transition, creating a much fairer difficulty curve. I still want the later battles to be challenging, but the challenge should come from making good decisions rather than from starting with a major statistical disadvantage.
Thank you for identifying the exact point where the progression becomes frustrating. This is very useful feedback for the next balancing pass!