I have every enemy card bar the stupid white mushroom card.I have killed over 160 white mushrooms using Warp as I read that is how you obtain the card. Am I doing it right or is there another method I should be using?
For this one, as I remember, it's really about luck. Warp aka the sleight that gets the entire field + soft-resetting after battles that didn't give you the enemy card, these things are meant to save you time (and White Room cards) as much as possible, but to get the actual card, it's all about luck. I remember my own struggles when I wanted to obtain all Enemy Cards and gee, it sure took a while.
So yes, you're doing it right, that is the method for obtaining the card. The rest is luck.