I'm not sure I know quite how to approach the daily challenge. Today's is to defeat Approach to Thistletop with only Ezren. Does that mean I need to take Ezren as a solo party through story mode until I get to and defeat that scenario? Or can I select just that scenario in some way? I'm not sure how this works.

I assume you haven't taken Ezren through story mode at all, correct? If not then this will probably be a pretty difficult challenge. If you have completed past Approach to Thistletop with Ezren in any party you can use that experienced character to create a new party and just select the scenario.

 

To get straight to AtT, put a new Ezren and a character that has completed AtT in the same party. Then go to manage party on the map screen, take out the experienced character and you should be able to select all the scenarios the experienced character has completed with the new Ezren.

I think that the game could update the available scenarios if you remove the experienced characters. If that way doesn't work, you have to complete the required scenario with Ezren and experienced characters before Daily challange, so Ez gets it in his completed ones. Then you can launch the solo quest for Ez.

If you don't have an experienced Ezren, I imagine you could form a party including Ezren, plug through Approach to Thistletop, and then create a new party with just that Experienced Ezren...