Save up them credits and rent cards. Don't buy packs, waste of money. Rent or buy single cards, and you'll be alright.
Had to learn this the hard way...after getting like 137 credits from a chest, only to buy the CL booster, only to get some bs, which i could have rented on the market.
sigh.
I've bought two packs, first pack I got a legendary and got excited so I bought another. HUGE flop was under the value of of the pack lol.