"even a relatively weak novice player could easily defeat opponents who had spent long months and a significant amount of investment in the game to build a strong deck"
Yes, exactly! That is one of the aspects that has made the fights in the game fun and exciting in the past!
"(which is wrong and unfair at least to strong players)."
Lol what? How can this possibly be unfair if the opponent is controlled by CPU and the HUMAN player whos deck was selected by the server neither actually plays nor knows anything about such fight taking place - and as such loses nothing at all?
"In addition, this meta prevented the potential for using active boosts and their effects,"
Maybe because those boost weren't such a great design in the first place? The damage increase from the boost is negligible, significant part of its duration is being wasted on waiting for monsters to respawn, chests to become openable, etc - so why bother using it at all? Instead you could have made much better sink for extra cards such as letting players exchange many lower level cards for a few higher level cards, or for gold or gems or items, etc.
"which negated our work in adding new content."
What content? The only content you are adding are new girl sets obtainable in exactly the same way as all the previous ones. How exactly old matchmaking and HP levels "negated your work" in adding such content?
"Battles in the upper leagues were ending in a couple of turns on average, making ... the battles boring."
And that is the second most important aspect of the fights that made them fun and exciting in the past! Its the LONG battles that are tedious and boring, not the short ones - and you were told this many times by different players on this forum!
"... making all the girls and all the buffs essentially the same ..."
This is so not true. Base dungeon girls have no perks - this alone makes them not equal to the event cards with perks. Cards with perks and better abilities made it possible in the past to win against player with much more HP or with higher base damage cards which also proves the cards were not the same. Now, with the HP values blown out of proportion, the advantage of the better ability cards in a long battle is lost because you can't "fast charge abilities - kill" your opponent anymore and as such the sets start to feel the same, because now I can only win by getting more base damage and more HP as opposed to instead getting better ability/perk cards. And do you know where this will lead? Here is a hint for you which you should care about - it will make it POINTLESS to spend MONEY on your game to buy event girls (for outrages prices I must add) because in the past when a new player bought a single mystic card he could immediately feel it: fast charge - bam - stronger opponent defeated, and now what does he get instead - here, spend 10 mins on a fight and then "enjoy" your "deserved" loss.
"As a result, we decided to adjust the HP of all characters. At the moment we are closely monitoring the duration of battles in the leagues and adjusting the matchmaking of opponents so that the duration of a fight with an equal opponent does not exceed 10 turns."
Tell me - has even a single player complained about battles before your HP update? And how many did complain after the update? And what your gameplay statistics showed after the update? What other reasons can you possibly need to understand that the pre-update balance was much better than it is now? 10 turns is still fucking way too long, a fight beyond 5 mins is way too long. Players want to be able to defeat decks with better abilities/perks by using shittier dungeon cards with leveled up base damage (by not allowing opponent to charge his cards - only possible in a SHORT fight!) or to defeat a player with much more HP and stronger deck by using low base damage deck with strong abilities/perks (by charging his cards first and one shotting the opponent, guess what - only possible in a SHORT fight!). Those are the things that make the fights truly fun and exciting.
So unless you realize what actually makes the game enjoyable and start making an effort to preserve all those fun aspect of the game - I am going back to playing something else.