I can't remember the name of the boss, but I think it was like a round greenish circle thing (I'm not sure how much that helps). It doesn't really have to have anything to do with the boss at all though, that was just what I could see that could be the issue ˙ᵕ˙
I think the issue with the cards disappearing quickly could also be fixed if there was a way to look through all the cards/pages you've collected. That way you could at least go to base and look through it there. That would prevent the gameplay from being interrupted in the heat of battle.