The bovine witch is way, way too strong. I can't win because no matter how many bovine potions I drag out, there's an infinite number taking their place. In deckbuilders - which this can be considered to be - cards whose only purpose to bloat the deck, i.e. do nothing but occupy a slot, are already very strong. It's very easy for such cards to spiral out of control. Adding a downside to playing the card or discarding it makes those cards even more powerful and difficult to deal with. By making it so you can't reroll the potions away, you've put them into impossible and broken territory. There's literally nothing you can do. You can't reroll, you can't discard them because an infinite number fill their place, and the more you try to get rid of the more are added to your deck because they duplicate every time they combine. The only way to win is to just not play, and reload the game as soon as you see three bovine potions on your shelf with an empty cauldron, because it's over.
I can only really see two ways to balance that witch and make it fun to play against and not a reload-fest. Either make it so the potions are removed when played, or make it so you can reroll them. As is, you're literally playing against the clock, and the witch gets the exact potions she needs way too often. It doesn't help that you only get two alternate potions before facing her, and 9 times out of 10 the potion you get will be completely useless. I've only beaten the bovine witch once, and it was by a complete fluke in the fact that I got the bovine potion so I filled her deck before she could fill mine.
Another issue is that the starting potions you get honestly aren't that strong. The expansion potion has its moments, but the only time you can use it effectively is when the witch's cauldron is already almost full, otherwise using it is an active detriment as it makes it easier for the witch to combine potions - especially apparent with the second witch with her instant-cast potions. I can also swear that the enemy witch's potions shrink. I've had many a time the cauldron is about to overflow, and all I need is one expansion potion, then the potions she has inexplicably shrink.