Cool game! I liked the mechanic where you can only have one permanent card, it made choosing which cards to take a lot more interesting and prevented using the same upgrades every attempt.
Holding down the attack button all the time seems to be quite powerful as then it attacks automatically when an enemy is near, meaning I didn't have to worry about enemies surprising me, this became even more powerful when I realized it was also automatically attacking and killing any projectiles fired at me. I became basically untouchable after combining this with strength and dexterity cards.