I rented those in the image:

But remove "Mermaid" and add lv4 "Molten Ogre" & ".Giant Roc" You'll notice I only rented 3 splinters. I also have good cards from the rewards like the water legendary, and lv3 Wood Nymph so I didn't rent those.

You can do the same with normal cards and they might even be cheaper, but I rented gold foils to earn more DEC per battle.