Instead of all Merfolk Shamans having exactly the same deck at decks/0150.dck, when a duel starts, choose one out of all files in decks/0150/*.dck.

Just picking a random file is easy; it'll go in TENTATIVE_SHANDALAR_read_enemy_deck(). The problem is ensuring that that's called exactly once per encounter - for all I know, it might read the deck before choosing an ante card, then read it again at duel start, etc. If so, maybe assign the random number to choose which deck to a global, and only generate a new random number at each new encounter.