I think that pretty much everybody will make the same deck for the Ray pod. It's all about the nuances and stuff like that as to who will make it out of there...
Just kidding. But seriously, none of these challenges are "impossible." About the hardest one is Armaldo in my opinion, and even that pales in comparison to...
Rayquaza, from EX: Emerald (yes, in my earlier days I actually GAVE that to people). Nowadays, I actually brainstorm the challenges I give people.
Seemingly easy cards or cards that already have known strategies are the hardest, imo.
"oh, that's so obvious to get a great decklist" and you would have like 10 similar decks, the difference down to techs, card type structure and the like.
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problem with armaldo is lack of heal in the format >_<