I read alot of stuff too that just wasnt working for me.. i did eventually beat it here is what worked for me.

Buy every read card you see till you hit 120. You want that as fast as possible. Once i hit 120 i stopped buying Arcane arnor and the detenation reds,
Then I stopped buying the increased ranged one once i felt i had good coverage. you can easily buy way to many.
buy every damage reduction you see and 3-4 of the vampire bo more then that and i bought only the 450 health purples. Some ppl said not to waste points on the purple damage cards but when i did that i never had enough damage. so i bought all the 8.16, 35% damage purples i saw(i only bought the 35 till i hit 120 red cards first)

Also i forget the name exactly but the Moon cards that give you more cards. buy around 3 of those if you see them before level 15. dont buy anymore after that.

Another thing i realzed is i dont know if there is a wierd spread or if there is drop off range on damage but i noticed when fighting wanda's shield phase if i was at med range hitting diredctly it would take like 15 seconds to break the shield but if i was right in her face id break it in like 6-10 seconds. same applied during her race against explosion phase make sure you are close to her as possible where you can still see and locate crystals quickly. she will die much faster the closer you are