Yeah. That's actually something I'm worried about; that the higher Leagues (especially Platinum) are just too frustrating to play. I'm pretty sure that even with those 5 copies of a card requirement I could make decks that are just about unbeatable, and even highly unfavourable if you take a direct counter. Just think about a beefed up Rainbow that has a better quanta balance, Supernovas and some superior choices, like Pulverizer instead of Eagle's Eye. Doesn't really matter which five cards he has to feature, the rest of the deck will still be insane.
An idea I had to possibly fix this: What if your level not only determined the deck you can submit but also your options when you go fight against the T500. So you might fight against triple mark decks but at least you have a triple mark of your own. This seems quite exciting as it would really add another dimension to PvE deckbuilding.
I have to say, I completely agree on this: compared to FG's (which sound to me like the ultimate opponent) the different arena decks are just way to hard, even compared to their reward. I mean it is just not fun to play a mirror match but with your opponent having a double draw and 200 hp's.
10 men's solution sound quite reasonable. To be honest I thought is would be like that (being, you build a deck to submit for top500, and when fighting top500 you get to use that same deck, including its buffs, instead of the normal deck you are using)
Another way to fix this, or maybe both can be used at the same time, would be to use the same skill point costs, but only give 1 extra skill point for each two levels (or simply using 10 levels for each league), that way, even the people in higher leagues have to chose what they improve, instead of just improving everything.