So it's no surprise for witch's tower that there are a lot of balancing issues (unscaled enemies, bad card pool, ect) but I think something that discussed enough is how bad the starting decks are.

First things first, to maximize energy use, you need to use 3 characters. This means 6 cards each which is 18 cards. An 18 deck is huge and really muddles deck potential, especially if you plan on acquiring even more cards.

That by itself is fine if it's an intentional design, but it leads to the larger issue in that there is barely any defense in the staring decks. Literately no character has a single defense card except Milla (Della has a weaken card and a misfortune card but its not defense). This means you have to run Milla since you will need some defense at least which hurts team building.

Even then Milla isn't the best at defending her team mates. Her passive is not guaranteed and of her staring 6 cards, only 2 of them can provide any defense to characters not herself (one card provides taunt which you can argue protects her teammates but taunt doesn't stop AOE damage which every boss and more common enemies has). This means you're trying to fish for 2 cards in an 18 card deck near the beginning (or more if you took cards). If you're praying to get defense in card pool picks, then 1. the stars need to align that its for the characters you're using 2. that the card itself is good 3. that you're filling up an already large 18 card deck.

Lastly, all upgrades don't fix this problem. Even upgrading Milla's situation assessment (the one card that protects allies) provides no additional defense to her teammates. No upgrade makes a normal card add defense. They almost all just add 2 more damage or add minor healing.

Personal solution: Each character should have a 5 card deck. Almost every starting deck has a clear card you can remove. Also each character should have a personal defense card to help reduce AOE damage.

there will still be issues (Zombies cant have 60 HP in the 1st round with these changes) but at least it will help with a lot of early damage.