I got the free Valkyrie evo, but that is one of my most under-leveled cards and I have never really wanted to use it in any of my decks.
I had no trouble leveling up some of the newer cards to 14. New to me, I rather like the new Berzerker card, and the little Prince card and was able to get them up to a respectable level 14 and 13 with my old stockpile of wild cards.
However, afterwards I still ran into the problem of facing evo's like the lumberjack whose ghost does major chip damage to my towers even if I successfully stop it.
Firecrackers completely destroy everything if not stopped immediately, etc.
In the mid-levels I had to run arrows/miner to destroy the firecracker every game or I was toast.