As someone that didn't play hunter all you need to do is get 750 crushing wins then you should have all treasures but if you want to get treasures faster here are my thoughts on how in arena.
I think you do get less chance in custom games.
You get a lot more chance if you play with people that dont have many treasures especially new players.
I dont think there is any difference between hard or crushing.
For some reason I also believe you get more chance if you use AO3 I dont know why.
All of these do not take in to account any rare treasure weekends that ND put up.
Please note I could be wrong and this is just for people that hate hunter as hunter is better.