I beat it pretty quick and my tactic was to first duplicate cards, then add a bunch of purity and health onto each one to make each very powerful, deleate some blsht ones and just then max out health on the characters to get the blessing cards. Don't try to keep all of them alive, I had 2 dead from almost very start, that's alright. I have a screen of the ending. Hope it helps:)
I had about 25 cards, I think?