Random under 30 cards against Rowena was nearly impossible for me, and I have no issue with a couple NPCs being so difficult early on. They made some NPCs a joke to beat with basic cards that rewarded 5 star cards, so that evens out. Getting odin wasn't bad and I did it with a 30 card deck quite easily. Rowena with a 30 card deck was about a 30% win rate and about 30% draws. Now that I have 60+ cards Rowena is about 50% win 25%/draw/loss. I only get Gerolts, but the difficult from "Random" drastically decreases with the rarity limit going up, at least once you figure out the AIs faults. Removing the rarity restriction for the rule would remove the need for anyone to get 30+ cards

Rowena is probably the only NPC that can't be farmed with a 70%+ win rate after you get 60 cards. They should have alternate decks for NPCs to use with stronger cards and better AI based on your deck size (with added MGP of course). Otherwise you are stuck with just Rowena for a challenge.