I think will keep the non-credit resource to a max of 2, and instead for the tier4 and 5 increase the credit costs. With the more powerful enemies you face at end of the game is not a problem to raise more credits anyway (I had some battles in which I maxed it!).
I'm also going to raise the HP of both heroes in those later battles, since with only 30HP the fight could last just a few minutes (some good card combo and you can easily end with a 5 attack unit that does double attack... and so
I've tested one which starts with both players at 50HP and was still probably just a few minutes (playing with max animation speed). Of course talking about Hard difficulty, Easy will still be very easy (and enemies will still have low HP, while your player higher one).
