It's kinda hidden from knowledge cause if you return here you don't find an option for an extra arena like the others. But talking to Thundermon changes the arena. Now the Junk Arena is even harder. This pretty much shows yellow cards true power.
Note: Probably the worse thing about this is trying to fight a Yellow deck with a Yellow Deck can get hard as a lot of decks feature cards that do multiplier damage to yellow type cards.
His deck features stronger discard option cards and lots of ways to abuse Crash.
Don't let him reach Ultimate level and you should be fine.
Oh dear god. Scary Yellow deck. First off if you use a dark or yellow deck probably rethink your life choices. 2nd he digivolves pretty fast and hits even harder. His deck is pretty up there in terms of being a hard to beat one.
Funny ending to it all lol.