Living walls is a card that only appears in certain AI decks. AI decks are associated with regions (eg cinderlands) not factions (eg fire) and level -- although the same AI deck is used with a given region for something like four levels in a row. Unfortunately, you can only defeat a given region once per level. If you gain a level, you can again play that region and there is a good chance you will get the same AI deck.
In an earlier thread (something to the effect of AI deck list), I attached a file which listed the power cards associated with each AI deck I was able to play. This thread is on page two of the forum as of today if you wanted to look and see where living walls arises.