Is there a way to increase your HP or the damage of your cards? After the first week everyone was up to the hundreds in health while I was sitting at abt 150 and spending what felt like hours chipping away at their health bars despite being similar sized or even larger than them
Demon deck is great for doing big damage. Been a while since I played so I can't remember the name of it, but one of them can do 250 - 500 damage. And some scale with your missing health. So if you have a card that boost your health to 2000 and then use one that buffs your dmg for the cost of health you can do 1000+ dmg sometimes! Just get all the demon cards and mess with 'em a bit. They're crazy stronk.
fairy fire decks are reliable method for scaling damage. The defender of the cosmos pack happens to to have a couple cards that not only increase their damage but also keep their damage increase between matches so if you can make a good deck from that pack early you can keep up your match winning speed without things slowing to a total crawl.
While cards that permanently grow your max hp exist the the booster is locked behind an absolutely diabolical side boss that'll make the current you just outright despair. There's a couple more resources for increasing hp though. There's a black market vendor at night time arena inside with 3 extremely useful items. Also win some gambling matches at the casino in order to access the VIP lounge which has a means of quickly increasing max hp if you're willing to sacrifice height.