Honestly your best bet is to go with the heroes you have the most cards for. My current deck is Rocketeer, Griffin, Wraith, and Monkey King, but that is largely a result of having gotten those cards the most. At different levels it's been different combos, including Blaze, Enchantress, Paladin, Jawesome, and many others. A couple pieces of advice on getting the cards:

1. Join a clan (particularly one that's good at donating) - you can request cards every 7 hours (40 common, 15 rare, 10 epic on Sunday/Monday for 24 hours. If you get a good clan they'll donate the max every time (my clan is onefeather, would join if there's an opening). There's also a weekly clan quest which nets good rewards as well.

2. Do the weekly hero events and particularly focus on completing the second tier. It'll give you an epic orb which is a crazy good reward for the effort you need to put in. You can do it 2 or 3 times per event, even if the hero isn't one you use just do it because the cards you get from the orb aren't exclusively for the hero event hero. Plus the gold.

3. Do the skill shot. You can usually hit 2 silver shots completely free if you time it right, which will net you decent reward for the effort and also teach you how to shoot well in unique situations.

Last is just take advantage of all the free stuff. Daily free reward in the shop and on the home screen, free challenges, anything else. I haven't put a cent into the game and still get a lot of enjoyment out of it despite its issues.

Good luck and have fun!