1. You need to have 2 ios devices logged into same account.
2. On your 1st device you play survivor mode and you try to get Nth metal as a reward. Pay one battle so you can claim your Nth metal when you cash out.
3. Take your 2nd device and buy all packs you want.
4. Logout from your main account on your 2nd device and login to another one. You can create a new account just for this.
5. When you logged to a new account on your 2nd device , cash out your Nth metal and credits in survivor mode on your 1st device.
6. Take your 2nd device and login back to your main account. Now you must have same credits and Nth metal on both devices. But on your 2nd device you also have all cards you bought. So if you want to sell them for making some profit you need
7. Logout from your main account on your 1st device and login to another one, probably new one you created just for this.
8. Sell some cards on your 2nd device.
9. Win a battle and press next.
10. Login to your main account on your 1st device. Now your devices are synced and you have same money and cards on itch of them.
Voila, good luck!