That's probably because it's an unused general? I'm fairly sure that means they never made a challenge mode for that one. You can't unlock content that doesn't exist.
Uhh...
Literally the first result if you google for General+Leang+Boss+General is a video that explains how to unlock him yourself... 😞
https://www.youtube.com/watch?v=m2oVUTsBD6Q
Given the fact this explains exactly how the modding happens, you can be 100% sure nothing is added that way, either.
This video doesn't show you how to get the Boss General in the Challenges Mode.
That's probably because it's an unused general? I'm fairly sure that means they never made a challenge mode for that one. You can't unlock content that doesn't exist.
When I tried to play this faction in the challenges mode it brought me back to the intro screen but none of the buttons appeared. But I guess you're right.
