There is indeed a (stickied) thread about overhauling the enemy decks. The part concerning Shandalar 2015 starts with the last post on
page 6 in the thread.guinsoo has made a full pack with decks for all of the enemies, stassy and quatrocentes-e-vinte have made decks for several of the enemies, and I and Fourth Inversion have made some, so there is already quite a bit to choose from. Read the overhaul thread for details. None have yet to include any cards that are new in the Thieves Hideout version Korath just released, though.
The enemy decks are stored in the Decks folder, with a (number).dck file for each creature. These can be opened and edited with Notepad.
Some of the people in the overhaul thread have provided downloads with one or several .dck files.
Extract these into the Decks folder, overwriting the existing one(s) if you want to play Shandalar with those enemy decks.
quatrocentos-e-vinte has made a spreadsheet (
this post) that provides a good overview of the enemy creatures, including which file numbers the respective decks have.
If you want to build some enemy decks yourself, the most convenient way is to use the Deck Builder. After saving a deck file (they are by default stored in the Playdeck folder), open that one and copy/paste the relevant parts over to the correct .dck file in the Decks folder.
To get the Deck Builder to show only cards available in the current version of Shandalar, make sure you have extracted the most recent rarity.dat file from Korath's download, then