I and many have said this. I think maybe arena was easy when the game started (don't know - new player) but certainly silver arena at Mission Stage 2 is pretty hard. Also it's kinda weird that they ask you for 500 silver tokens (or whatever) then ask you to get to Silver arena later (doesn't make sense).  Then ask you to beat campaign on hard. (lol)

So, completely agree, but actually if you can do this, it sets you up well for Dungeon 15-17-20 later. Because sooner or later, you need to gear up characters to that kind of level. There is also Great Hall 10 mission later ;-)

Don't worry about the mission order. It's nice when you finally pass a hurdle (this is just the first) and you walk through the next 10 or more. Make your decision now on which way to go with great hall.  I recommend RES or ACC depending on your team makeup.