how to get up in Hearthstone?

Hey guys,

So I just bought a beta key, and I've gotten all heros to level 10, and beaten all the CPU heros on expert mode.
I used the gold on arena but the most kills I've gotten per arena are two. I never get any good cards in the packs either

Was wondering what I should do next to get good cards (legendaries)

Thanks.

Only way to get cards is to either: buy packs with gold/money, crafting them, leveling up the hero's or getting them as rewards.