The Order of Arena-Related Progress Missions Should Be Changed
I don't think new players should worry about reaching Bronze III until after they've finished the Campaign Mission on Hard. Reaching Silver can wait until they're finished with Brutal. The Mission for getting 30 silver medals shouldn't appear until they're a quarter of the way through Nightmare. Halfway through Nightmare they can get a Mission for reaching Gold tier. Getting medals in Gold Tier can wait until they're 75% through Nightmare. Then maybe they can get a Mission for reaching Platinum after they've cleared Nightmare.
Perhaps the order of the Missions made sense when the game was first released. However, though I'm a new player, I'd wager that the Arena has become significantly more challenging since then.