by Bartheus
Even though the question was for Peter, I hope you don't mind if I chime in.In my opinion, the game is lot harder when played with one hero than with any other number of heroes. Even though the game has some built-in balancing, like area of effect attacks and encounters and increased monster activations when multiples are in play, they are not enough. In a typical scenario you have to explore 9-12 tiles and then kill a villain. For a solo hero, that of course means that the solo hero has to survive all that, taking at least that many rounds of play for exploration alone.
For three heroes, each hero individually only has to survive one third of those rounds to get to the villain. And even though the villain acts after each hero, it is a lot faster to kill the villain with three Dailies instead of one (if those Daily Powers are still unused at that time).
Granted, I am guessing that this is exactly the reason why the game lists one adventure as suitable for solo play and the rest as group scenarios. If you say that an adventure lasts for 12 hero-rounds, that means that for 2 heroes it is six rounds, for three heroes four rounds, for four three and so on. Here the change in the amount of rounds/hero is no longer that significant, allowing the aforementioned balancing to work more properly. I still think that with two heroes it is harder than with 3-5, but after that I am not so sure how it goes.