One good tip for earning XP is to play on Insane whenever you are full on health, and especially when "exploring" (going places for the first time)...xp goes up at a nice clip on insane as you go square to square, and if you hit a snag (a fight a little too hard) simply hit start/ change diff/ normal (or casual...I stick to normal to keep getting SOME xp!). My brother and I played the same amount of time, he is at 50% map, on Normal, at lvl 10, I am at 36% map, mostly Insane, and am lvl 16. Boss fights also get crazy xp on Insane, and if there is a save just prior, only takes a couple of tries.
As for Proving Grounds, I saw it as more of an expanded Tutorial (if you do the tutorial it wraps right into the packs), and teaching you techniques and "tricks" to the game, more than challenge levels you needed to play the game to master. I think the 3rd pack is a little tricky no matter when you do it!
I would list my completed games, but there just isn't enough room in the window.