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is it pretty much impossible to win solo with just Dragonspire? And do other sets make it more feasible?
Hah! Great question.
My gut instinct is that it is harder with just the Dragonspire cards. I haven't tried this just yet but I will over the next week and will come back to share some direct experience feedback.
What I can say now is that with all of the expansions, I typically win around 2 out of every 3 games. That ratio is higher if I play with the solo rules in Wrath of the Elements which is a bit easier as you have more cards to work with and don't have any leveling up requirements. In my experience, the hardest part of the Dragonspire solo rules is in pacing your dungeon/village visits to make sure you hit your leveling up requirement before the Thunderstone is drawn.
If this issue is nailing you as well OR if you want to reduce the challenge a notch, I suggest adding a 4th monster group. This would eliminate some of the race as the extra cards and time would make a world of difference in having time to level up. It would also give you more fun fight time with your level 3 heroes.
I also suggest always playing with two Thuderstones mixed into the bottom 10 cards. I always do this (solo or in groups) as seeing the first Thunderstone is kind of a "last call" single that adds some nice tension to the final rounds and eliminates having to pay as close of attention to the amount of remaining cards.