Maybe I missed it, but is there a mechanic or advice for playtesting with a solo character? I seem to recall it being mentioned that Mythic rules would include an option for single PCs.
Now these are threads from August that mention the goal of solo play, but maybe that goal was dropped?
Might be something they'll post here on the boards. I know James said he wanted to stat up a couple of CR 36 challenges and post them, so maybe some kind of advice for trying out solo play is forthcoming.
Or...you might just tinker around on your own and see what a solo character can handle. I'd just estimate the CR of your solo PC and put together an encounter with a CR equal to the PC -2 or -3.
And it might be something even they're not sure about yet until they get some play test reports in.
I think a solo game with mythic rules would work best with a character who has the Leadership feat and is a versatile class or has the ability scores to back it up. That way the player can play solo with support.