what actually you mean by upgrading the deck? Do you mean buying new cards or leveling up the cards I already have? In the case of buying cards, I think renting is the best option so I rent cards for a week or more.
And I have cards with 2,3 & 4 levels but I couldn't use those cards as their level because my summoners are not upgraded. I think upgrading the summoners will be a good decision.
Thank you for your point of view @zaku