I think that this is probably a question for the FFG message boards where you'll get some rules experts to chime in.
I think you would be able to do a challenge in the epic phase. The rules text of Focused Offense allows you to only "declare" a single challenge. However the text of Battle of Ruby ford allows you to "initiate" a challenge, implying that it's a player action instead of a challenge. But I'm not 100% sure on that.
The ability to initiate does not bypass the need to declare a target of a challenge, but with Focused Offense, you can't.