There are issues on both the Challenge and your decks :
Originally Archenemy was EDH Challenge, recently it was modified to be the following
"Enter the Dragon" Challenge is now "Archenemy's Cohort" : beat the 8 powerful, legendary allies of the Archenemy and then the Archenemy itself ! Your reward : the Archenemy AVATAR, courtesy of Jatill.
The deck-building rules are similar to the previous, but with a main difference : your deck must be at least 100 cards and every card is restricted to one, except for basic lands.
- Using deck archenemy bant or esper does not trigger the challenge at all and use normal Gauntlet mode settings
- deck archenemy jund has several non coded cards like
Soul Reap or Count Humunculus.
I though it came from a non updated Magic playdeck so I ran a PDA on it and...it triggered the challenge
The problem is the challenge is no longer a challenge because...the Archenemy card is on the player side instead of the AI side (maybe because I won the challenge before but in that case there should be Archenemy on both side)
Anyway confirming there is an issue.
I attached the corrected archenemy jund deck, I don't know why the challenge doesn't trigger for your others 2 decks, PDA didn't notice anything about them.