As it is,
its really rare and difficult to get a nymph,
but they are amazing cards.
What if we added a really difficult quest,
maybe where you had to go through a series of difficult battles to get one Nymph.
And you can redo the quest, but each time you do it the difficulty increases dramatically (to prevent farming)
And maybe when you go to upgrade a nymph, give the option of either upgrading it, or changing its element
(but make it even more expensive than upgrading, or if you change the element it loses its upgrade).
That way if you have a nymph, you aren't necessarily stuck using that element.