"Music That Glows In The Dark"
It's co op. So solo means play multiple characters. Solitaire would be one character play and given that the interaction of card combos between multiple heroes is a major aspect of the design, would probably not work well.
This game should not be difficult to play solo. In fact, with full control it may actually be much more tactical then in multiplayer.
It certainly would not require the sort of two brained thinking that soloing a standard wargame requires.
I just worry it may be overwhelming trying to manage two or more deck of cards.