Where is the 1st hub boss/sub-boss? (Possible Spoilers)

[KingKRool2002]

I completed all of the Challenges in the 1st hub area, but I do forgot to collect the boss key & now it isn't there anymore. Also, I can't find the boss/sub-boss portal anywhere in that hub. I already beat Metal Sonic & I also have most of the 2nd hub Challenges done, but I still have no idea how to fight the
1st hub boss. Did I encounter a glitch? Any help with this would be greatly appreciated. Also, could someone explain the exact requirements for getting the key in a hub?

Thanks.

[Lital]

Every zone has several challenges, you fight a boss at the end of every 3 zones, so after Sky Sanctuary would be the 1st real boss. You retrieve 3 keys prior to fighting the boss by doing 1 of the 5 or so challenges that show up once you finish Acts 1 and 2 of a zone.

[KingKRool2002]

Okay, thanks for the info. Is there any point in doing more than 1 Challenge in a zone?

[Shawn5150]

For fun, also you gain some skills at the shop for completing some challenges.