You need the solo deck- the rules scale against you after you win your first few battles as well. It's worthwhile but nearly any AI rule set will do well enough. It's very playable solo.
I use the solo deck to practice mostly and stay rules sharp over the summer months on rainy days. (Strange Aeons is Scheduled for Fall, Frostgrave in the Spring, Camping and Fishing in the Summer, Boardgames in the Winter.)